The Subjection of Women is an essay by English philosopher, political economist and civil servant John Stuart Mill published in 1869, with ideas he developed jointly with his wife Harriet Taylor Mill. Mill submitted the finished manuscript of their collaborative work On Liberty (1859) soon after her untimely death in late 1858, and then continued work on The Subjection of Women
until its completion in 1861. At the time of its publication, the
essay's argument for equality between the sexes was an affront to
European conventional norms regarding the status of men and women.
In his Autobiography, Mill describes his indebtedness to his wife, and his daughter Helen Taylor for the creation of The Subjection of Women:
As ultimately published it was
enriched with some important ideas of my daughter’s and some passages of
her writing. But all that is most striking and profound in what was
written by me belongs to my wife, coming from the fund of thought that
had been made common to us both by our innumerable conversations and
discussions on a topic that filled so large a place in our minds.
While scholars generally agree that John Stuart Mill was the sole author, it is also noted that some of the arguments are similar to Harriet Taylor Mill's essay The Enfranchisement of Women, which was published in 1851.
Mill was convinced that the moral and intellectual advancement of
humankind would result in greater happiness for everybody. He asserted
that the higher pleasures of the intellect yielded far greater happiness
than the lower pleasure of the senses. He conceived of human beings as
morally and intellectually capable of being educated and civilised. Mill
believed everyone should have the right to vote, with the only
exceptions being barbarians and uneducated people.
Mill argues that people should be able to vote to defend their
own rights and to learn to stand on their two feet, morally and
intellectually. This argument is applied to both men and women. Mill
often used his position as a member of Parliament to demand the vote for
women, a controversial position for the time.
In Mill's time a woman was generally subject to the whims of her
husband or father due to social norms which said women were both
physically and mentally less able than men and therefore needed to be
"taken care of". Contributing to this view were both hierarchical
religious views of men and women within the family and social theories
based on biological determinism. The archetype of the ideal woman as mother, wife and homemaker was a powerful idea in 19th century society.
At the time of writing, Mill recognized that he was going against
the common views of society and was aware that he would be forced to
back up his claims persistently. Mill argued that the inequality of
women was a relic from the past, when "might was right," but it had no place in the modern world.
Mill saw that having effectively half the human race unable to
contribute to society outside of the home was a hindrance to human
development.
... [T]he legal subordination of
one sex to another – is wrong in itself, and now one of the chief
hindrances to human improvement; and that it ought to be replaced by a
system of perfect equality, admitting no power and privilege on the one
side, nor disability on the other.
Arguments
Mill attacks the argument that women are naturally worse at some
things than men and should, therefore, be discouraged or forbidden from
doing them. He says that we simply don't know what women are capable of,
because we have never let them try – one cannot make an authoritative
statement without evidence. We can't stop women from trying things
because they might not be able to do them. An argument based on
speculative physiology is just that, speculation.
The anxiety of mankind to intervene
on behalf of nature...is an altogether unnecessary solicitude. What
women by nature cannot do, it is quite superfluous to forbid them from
doing.
In this, men are basically contradicting themselves because they say
women cannot do an activity and want to stop them from doing it. Here
Mill suggests that men are basically admitting that women are capable of
doing the activity, but that men do not want them to do so.
Whether women can do them or not must be found out in practice.
In reality, we don't know what women's nature is, because it is so
wrapped up in how they have been raised. Mill suggests we should test
out what women can and can't do – experiment.
I deny that any one knows or can
know, the nature of the two sexes, as long as they have only been seen
in their present relation to one another. Until conditions of equality
exist, no one can possibly assess the natural differences between women
and men, distorted as they have been. What is natural to the two sexes
can only be found out by allowing both to develop and use their
faculties freely.
Women are brought up to act as if they were weak, emotional, docile –
a traditional prejudice. If we tried equality, we would see that there
were benefits for individual women. They would be free of the
unhappiness of being told what to do by men. And there would be benefits
for society at large – it would double the mass of mental faculties
available for the higher service of humanity. The ideas and potential of
half the population would be liberated, producing a great effect on
human development.
Mill's essay is clearly utilitarian in nature on three counts: The immediate greater good, the enrichment of society, and individual development.
If society really wanted to discover what is truly natural in gender relations, Mill argued, it should establish a free market
for all of the services women perform, ensuring a fair economic return
for their contributions to the general welfare. Only then would their
practical choices be likely to reflect their genuine interests and
abilities.
Mill felt that the emancipation
and education of women would have positive benefits for men also. The
stimulus of female competition and companionship of equally educated
persons would result in the greater intellectual development of all. He
stressed the insidious effects of the constant companionship of an
uneducated wife or husband. Mill felt that men and women married to
follow customs and that the relation between them was a purely domestic
one. By emancipating women, Mill believed, they would be better able to
connect on an intellectual level with their husbands, thereby improving
relationships.
Mill attacks marriage laws, which he likens to the slavery of
women, "there remain no legal slaves, save the mistress of every house."
He alludes to the subjection of women becoming redundant as slavery
did before it. He also argues for the need for reforms of marriage
legislation whereby it is reduced to a business agreement, placing no
restrictions on either party. Among these proposals are the changing of
inheritance laws to allow women to keep their own property, and allowing
women to work outside the home, gaining independent financial
stability.
Again the issue of women's suffrage
is raised. Women make up half of the population, thus they also have a
right to a vote since political policies affect women too. He theorises
that most men will vote for those MPs who will subordinate women,
therefore women must be allowed to vote to protect their own interests.
Under whatever conditions, and
within whatever limits, men are admitted to the suffrage, there is not a
shadow of justification for not admitting women under the same.
Mill felt that even in societies as unequal as England and Europe
that one could already find evidence that when given a chance women
could excel. He pointed to such English queens as Elizabeth I, or Victoria, or the French patriot, Joan of Arc. If given the chance women would excel in other arenas and they should be given the opportunity to try.
Mill was not just a theorist; he actively campaigned for women's rights as an MP and was the president of the National Society for Women's Suffrage.
Conclusions
The
way Mill interpreted subjects over time changed. For many years Mill
was seen as an inconsistent philosopher, writing on a number of separate
issues. Consistency in his approach is based on utilitarianism, and the
good of society.
Utilitarianism
Nothing
should be ruled out because it is just "wrong" or because no one has
done it in the past. When we are considering our policies, we should
seek the greatest happiness of the greatest number. This leads to
attacks on conventional views. If you wish to make something illegal,
you need to prove what harm is being done. Individuals know their own
interests best.
Progress of society
The
greatest good is understood in a very broad sense to be the moral and
intellectual developments of society. Different societies are at
different stages of development or civilisation. Different solutions may
be required for them. What matters is how we encourage them to advance
further. We can say the same for individuals. Mill has a quite specific
idea of individual progress: (1) employing higher faculties; (2) moral
development, with people placing narrow self-interest behind them.
Individual self-reliance
We
are independent, capable of change and of being rational. Individual
liberty provides the best route to moral development. As we develop, we
are able to govern ourselves, make our own decisions, and not to be
dependent on what anyone else tells us to do. Democracy is a form of
self-dependence.
This means:
Personal Liberty As long as we do not harm others, we should be able to express our own natures, and experiment with our lives
Liberty to Govern our own Affairs Civilized people are
increasingly able to make their own decisions, and protect their own
rights. Representative government is also a useful way of getting us to
think about the common good.
Liberty for women as well as men All of Mill's arguments
apply to both men and women. Previous ideas about the different natures
of men and women have never been properly tested. Women can participate
in determining their own affairs too.
On Liberty is a philosophical essay by the English philosopherJohn Stuart Mill. Published in 1859, it applies Mill's ethical system of utilitarianism to society and state. Mill suggests standards for the relationship between authority and liberty. He emphasizes the importance of individuality, which he considers prerequisite to the higher pleasures—the summum bonum of utilitarianism. Furthermore, Mill asserts that democratic ideals may result in the tyranny of the majority.
Among the standards proposed are Mill's three basic liberties of
individuals, his three legitimate objections to government intervention,
and his two maxims regarding the relationship of the individual to
society.
On Liberty was a greatly influential and well-received work. Some classical liberals and libertarians have criticized it for its apparent discontinuity with Utilitarianism,
and vagueness in defining the arena within which individuals can
contest government infringements on their personal freedom of action.
The ideas presented in On Liberty have remained the basis of much political thought. It has remained in print since its initial publication. A copy of On Liberty is passed to the president of the British Liberal Democrats as a symbol of office.
Mill's marriage to Harriet Taylor Mill greatly influenced the concepts in On Liberty, which was published shortly after she died.
Composition
According to Mill in his autobiography, On Liberty
was first conceived as a short essay in 1854. As the ideas developed,
the essay was expanded, rewritten and "sedulously" corrected by Mill and
his wife, Harriet Taylor.
Mill, after suffering a mental breakdown and eventually meeting and
subsequently marrying Harriet, changed many of his beliefs on moral life
and women's rights. Mill states that On Liberty "was more directly and literally our joint production than anything else which bears my name."
The final draft was nearly complete when his wife died suddenly in 1858.
Mill suggests that he made no alterations to the text at this point and
that one of his first acts after her death was to publish it and to
"consecrate it to her memory." The composition of this work was also indebted to the work of the German thinker Wilhelm von Humboldt, especially his essay On the Limits of State Action. Finally published in 1859, On Liberty was one of Mill's two most influential books (the other being Utilitarianism).
Overview
Introduction
John Stuart Mill opens his essay by discussing the historical "struggle between authority and liberty," describing the tyranny
of government, which, in his view, needs to be controlled by the
liberty of the citizens. He divides this control of authority into two
mechanisms: necessary rights belonging to citizens, and the
"establishment of constitutional checks by which the consent of the
community, or of a body of some sort, supposed to represent its
interests, was made a necessary condition to some of the more important
acts of the governing power."
Because society was—in its early stages—subjected to such turbulent
conditions (i.e. small population and constant war), it was forced to
accept rule "by a master."
However, as mankind progressed, it became conceivable for the people to
rule themselves. Mill admits that this new form of society seemed
immune to tyranny because "there was no fear of tyrannizing over self." Despite the high hopes of the Enlightenment,
Mill argues that the democratic ideals were not as easily met as
expected. First, even in democracy, the rulers were not always the same
sort of people as the ruled. Second, there is a risk of a "tyranny of the majority"
in which the many oppress the few who, according to democratic ideals,
have just as much a right to pursue their legitimate ends.
In Mill's view, tyranny of the majority is worse than tyranny of
government because it is not limited to a political function. Where one
can be protected from a tyrant, it is much harder to be protected
"against the tyranny of the prevailing opinion and feeling."
The prevailing opinions within society will be the basis of all rules
of conduct within society; thus there can be no safeguard in law against
the tyranny of the majority. Mill's proof goes as follows: the majority
opinion may not be the correct opinion. The only justification for a
person's preference for a particular moral belief is that it is that
person's preference. On a particular issue, people will align themselves
either for or against that issue; the side of greatest volume will
prevail, but is not necessarily correct.
In conclusion to this analysis of past governments, Mill proposes a
single standard for which a person's liberty may be restricted:
That the only purpose for which power can be rightfully
exercised over any member of a civilized community, against his will, is
to prevent harm to others. His own good, either physical or moral, is
not a sufficient warrant ... Over himself, over his body and mind, the
individual is sovereign.
Mill clarifies that this standard is solely based on utility, not on natural rights. According to Mill, children and "barbarian" nations are benefited by limited freedom. Just despots, such as Charlemagne and Akbar the Great, were historically beneficial to people not yet fit to rule themselves.
J. S. Mill concludes the Introduction by discussing what he claimed were the three basic liberties in order of importance:
The freedom of thought and emotion. This includes the freedom to act on such thought, i.e. freedom of speech
The freedom to pursue tastes (provided they do no harm to others), even if they are deemed "immoral"
The freedom to unite so long as the involved members are of age, the
involved members are not forced, and no harm is done to others
While Mill admits that these freedoms could—in certain situations—be
pushed aside, he claims that in contemporary and civilised societies
there is no justification for their removal.
Of the liberty of thought and discussion
In the second chapter, J. S. Mill attempts to prove his claim from the first chapter that opinions ought never to be suppressed.
Looking to the consequences of suppressing opinions, he concludes that
opinions ought never to be suppressed, stating, "Such prejudice, or
oversight, when it [i.e. false belief] occurs, is altogether an evil;
but it is one from which we cannot hope to be always exempt, and must be
regarded as the price paid for an inestimable good." He claims that
there are three sorts of beliefs that can be had—wholly false, partly
true, and wholly true—all of which, according to Mill, benefit the
common good:
First, if any opinion is compelled
to silence, that opinion may, for aught we can certainly know, be true.
To deny this is to assume our own infallibility. Secondly, though the
silenced opinion be an error, it may, and very commonly does, contain a
portion of truth; and since the general or prevailing opinion on any
subject is rarely or never the whole truth, it is only by the collision
of adverse opinions that the remainder of the truth has any chance of
being supplied. Thirdly, even if the received opinion be not only true,
but the whole truth; unless it is suffered to be, and actually is,
vigorously and earnestly contested, it will, by most of those who
receive it, be held in the manner of a prejudice, with little
comprehension or feeling of its rational grounds. And not only this,
but, fourthly, the meaning of the doctrine itself will be in danger of
being lost, or enfeebled, and deprived of its vital effect on the
character and conduct: the dogma becoming a mere formal profession,
inefficacious for good, but cumbering the ground, and preventing the
growth of any real and heartfelt conviction, from reason or personal
experience.
Mill spends a large portion of the chapter discussing implications of
and objections to the policy of never suppressing opinions. In doing so, Mill explains his opinion of Christian ethics, arguing that, while they are praiseworthy,
they are incomplete on their own. Therefore, Mill concludes that
suppression of opinion based on belief in infallible doctrine is
dangerous. Among the other objections Mill answers is the objection that the truth will necessarily survive persecution and that society need only teach the grounds for truth, not the objections to it.
Near the end of Chapter 2, Mill states that "unmeasured vituperation,
enforced on the side of prevailing opinion, deters people from
expressing contrary opinion, and from listening to those who express
them."
On individuality as one of the elements of well-being
In the third chapter, J. S. Mill points out the inherent value of individuality since individuality is ex vi termini (i.e. by definition) the thriving of the human person through the higher pleasures. He argues that a society ought to attempt to promote individuality as it is a prerequisite for creativity and diversity.
With this in mind, Mill believes that conformity is dangerous. He states that he fears that Western civilization approaches this well-intentioned conformity to praiseworthy maxims characterized by the Chinese civilization.
Therefore, Mill concludes that actions in themselves do not matter.
Rather, the person behind the action and the action together are
valuable. He writes:
It really is of importance, not
only what men do, but also what manner of men they are that do it. Among
the works of man, which human life is rightly employed in perfecting
and beautifying, the first in importance surely is man himself.
Supposing it were possible to get houses built, corn grown, battles
fought, causes tried, and even churches erected and prayers said, by
machinery—by automatons in human form—it would be a considerable loss to
exchange for these automatons even the men and women who at present
inhabit the more civilised parts of the world, and who assuredly are but
starved specimens of what nature can and will produce. Human nature is
not a machine to be built after a model, and set to do exactly the work
prescribed for it, but a tree, which requires to grow and develop itself
on all sides, according to the tendency of the inward forces which make
it a living thing.
On the limits to the authority of society over the individual
In
the fourth chapter, J. S. Mill explains a system in which a person can
discern what aspects of life should be governed by the individual and
which by society.
Generally, he holds that a person should be left as free to pursue his
own interests as long as this does not harm the interests of others. In
such a situation, "society has jurisdiction over [the person's
conduct]."
He rejects the idea that this liberty is simply for the purpose of
allowing selfish indifference. Rather, he argues that this liberal
system will bring people to the good more effectively than physical or
emotional coercion.
This principle leads him to conclude that a person may, without fear of
just punishment, do harm to himself through vice. Governments, he
claims, should only punish a person for neglecting to fulfill a duty to
others (or causing harm to others), not the vice that brought about the
neglect.
J. S. Mill spends the rest of the chapter responding to
objections to his maxim. He notes the objection that he contradicts
himself in granting societal interference with youth because they are
irrational but denying societal interference with certain adults though
they act irrationally.
Mill first responds by restating the claim that society ought to punish
the harmful consequences of the irrational conduct, but not the
irrational conduct itself which is a personal matter. Furthermore, he notes the societal obligation is not to ensure that each individual is moral throughout adulthood.
Rather, he states that, by educating youth, society has the opportunity
and duty to ensure that a generation, as a whole, is generally moral.
Where some may object that there is justification for certain
religious prohibitions in a society dominated by that religion, he
argues that members of the majority ought make rules that they would
accept should they have been the minority.
He states, "unless we are willing to adopt the logic of persecutors,
and say that we may persecute others because we are right, and that they
must not persecute us because they are wrong, we must beware of
admitting a principle of which we should resent as a gross injustice the
application to ourselves."
In saying this, he references an earlier claim that morals and religion
cannot be treated in the same light as mathematics because morals and
religion are vastly more complex.
Just as with living in a society which contains immoral people, Mill
points out that agents who find another's conduct depraved do not have
to socialise with the other, merely refrain from impeding their personal
decisions.
While Mill generally opposes the religiously motivated societal
interference, he admits that it is conceivably permissible for
religiously motivated laws to prohibit the use of what no religion
obligates. For example, a Muslim state could feasibly prohibit pork.
However, Mill still prefers a policy of society minding its own
business.
Applications
This last chapter applies the principles laid out in the previous sections. He begins by summarising these principles:
The maxims are, first, that the
individual is not accountable to society for his actions, in so far as
these concern the interests of no person but himself. Advice,
instruction, persuasion, and avoidance by other people if thought
necessary by them for their own good, are the only measures by which
society can justifiably express its dislike or disapprobation of his
conduct. Secondly, that for such actions as are prejudicial to the
interests of others, the individual is accountable, and may be subjected
either to social or to legal punishment, if society is of opinion that
the one or the other is requisite for its protection.
Economy
Mill
first applies these principles to the economy. He concludes that free
markets are preferable to those controlled by governments. While it may
seem, because "trade is a social act," that the government ought
intervene in the economy, Mill argues that economies function best when
left to their own devices. Therefore, government intervention, though theoretically permissible, would be counterproductive.
Later, he attacks government-run economies as "despotic." He believes
that if the government ran the economy, then all people would aspire to
be part of a bureaucracy that had no incentive to further the interests
of any but itself.
Preventing harm
Next Mill investigates in what ways a person may try to prevent harm.
He first admits that a person should not wait for injury to happen, but
ought try to prevent it. Second, he states that agents must consider
whether that which can cause injury can cause injury exclusively.
He gives the example of selling poison. Poison can cause harm. However,
he points out that poison can also be used for good. Therefore, selling
poison is permissible.
Yet, due to the risk entailed in selling poison or like products (e.g.
alcohol), he sees no danger to liberty to require warning labels on the
product.
Again, Mill applies his principle. He considers the right course of
action when an agent sees a person about to cross a condemned bridge
without being aware of the risk. Mill states that because the agent
presumably has interest in not crossing a dangerous bridge (i.e. if he
knew the facts concerned with crossing the bridge, he would not desire
to cross the bridge), it is permissible to forcibly stop the person from
crossing the bridge. He qualifies the assertion stating that, if the
means are available, it is better to warn the unaware person.
With regard to taxing to deter agents from buying dangerous
products, he makes a distinction. He states that to tax solely to deter
purchases is impermissible because prohibiting personal actions is
impermissible and "[e]very increase of cost is a prohibition, to those
whose means do not come up to the augmented price."
However, because a government must tax to some extent in order to
survive, it may choose to take its taxes from what it deems most
dangerous.
Repeat offences to public through private action
Mill
expands upon his principle of punishing the consequences rather than
the personal action. He argues that a person who is empirically prone to
act violently (i.e. harm society) from drunkenness (i.e. a personal
act) should be uniquely restricted from the drinking. He further
stipulates that repeat offenders should be punished more than first time
offenders.
Encouraging vice
On the subject of fornication and gambling, Mill has no conclusive answer, stating, "[t]here are arguments on both sides."
He suggests that while the actions might be "tolerated" in private,
promoting the actions (i.e. being a pimp or keeping a gambling house)
"should not be permitted." He reaches a similar conclusion with acts of indecency, concluding that public indecency is condemnable.
Suicide and divorce
Mill
continues by addressing the question of social interference in suicide.
He states that the purpose of liberty is to allow a person to pursue
their interest. Therefore, when a person intends to terminate their
ability to have interests it is permissible for society to step in. In
other words, a person does not have the freedom to surrender their
freedom. To the question of divorce, Mill argues that marriages are one of the most important structures within society;
however, if a couple mutually agrees to terminate their marriage, they
are permitted to do so because society has no grounds to intervene in
such a deeply personal contract.
Education
Mill
believes that government run education is an evil because it would
destroy diversity of opinion for all people to be taught the curriculum
developed by a few. The less evil version of state run schooling, according to Mill, is that which competes against other privately run schools.
In contrast, Mill believes that governments ought to require and fund
private education. He states that they should enforce mandatory
education through minor fines and annual standardised testing that
tested only uncontroversial fact. He goes on to emphasise the importance of a diverse education that teaches opposing views (e.g. Kant and Locke).
He concludes by stating that it is legitimate for states to forbid
marriages unless the couple can prove that they have "means of
supporting a family" through education and other basic necessities.
Conclusion
J. S. Mill concludes by stating three general reasons to object to governmental interference:
if agents do the action better than the government.
if it benefits agents to do the action though the government may be more qualified to do so.
if the action would add so greatly to the government power that it
would become over-reaching or individual ambition would be turned into
dependency on government.
He summarises his thesis, stating:
The worth of a State, in the long
run, is the worth of the individuals composing it; and a State which
postpones the interests of their mental expansion and elevation,
to a little more of administrative skill, or of that semblance of it
which practice gives, in the details of business; a State which dwarfs
its men, in order that they may be more docile instruments in its hands
even for beneficial purposes—will find that with small men no great
thing can really be accomplished; and that the perfection of machinery
to which it has sacrificed everything, will in the end avail it nothing,
for want of the vital power which, in order that the machine might work
more smoothly, it has preferred to banish.
Reception
On Liberty was enormously popular in the years following its publication. Thomas Hardy recalled later in life that undergraduates in the 1860s knew the book almost by heart.
Criticisms of the book in the 19th century came chiefly from thinkers
who felt that Mill's concept of liberty left the door open for
barbarism, such as James Fitzjames Stephen and Matthew Arnold.
In more recent times, although On Liberty garnered adverse criticism, it has been largely received as an important classic
of political thought for its ideas and accessibly lucid style. Denise
Evans and Mary L. Onorato summarise the modern reception of On Liberty, stating: "[c]ritics regard his essay On Liberty
as a seminal work in the development of British liberalism. Enhanced by
his powerful, lucid, and accessible prose style, Mill's writings on
government, economics, and logic suggest a model for society that
remains compelling and relevant." As one sign of the book's importance, a copy of On Liberty is the symbol of office for the president of the Liberal Democrat Party in England.
Contradiction to utilitarianism
Mill makes it clear throughout On Liberty
that he "regard[s] utility as the ultimate appeal on all ethical
questions", a standard he inherited from his father, a follower of Jeremy Bentham.
Though J. S. Mill claims that all of his principles on liberty appeal
to the ultimate authority of utilitarianism, according to Nigel Warburton,
much of the essay can seem divorced from his supposed final court of
appeals. Mill seems to idealize liberty and rights at the cost of
utility. For instance, Mill writes:
If all mankind minus one, were of
one opinion, and only one person were of contrary opinion, mankind would
be no more justified in silencing that one person, than he, if he had
the power, would be justified in silencing mankind.
This claim seems to go against the principle of utilitarianism, that
it is permissible that one should be harmed so that the majority could
benefit.
Warburton argues that Mill is too optimistic about the outcome of
free speech. Warburton suggests that there are situations in which it
would cause more happiness to suppress truth than to permit it. For
example, if a scientist discovered a comet about to kill the planet in a
matter of weeks, it may cause more happiness to suppress the truth than
to allow society to discover the impending danger.
While David Brink concedes that Mill's apparently categorical
appeal to rights seems to contradict utilitarianism, he points out that
Mill does not believe rights are truly categorical because Mill opposes
unrestrained liberty (e.g. offensive public exposure).
Furthermore, David Brink tries to reconcile Mill's system of rights with utilitarianism in three ways:
Rights are secondary principles to the Greatest Happiness Principle
Rights are incomparable goods, justifying their categorical enforcement
Liberty is a good. Thus, those who suppress it are worthy of
punishment. Rights deal with the value of punishing/protecting others'
interference with liberty, not the actual protection of liberty.
Narrow focus
Some
thinkers have criticised Mill's writing for its apparent narrow or
unclear focus in several areas. Mill makes clear that he only considers
adults in his writing, failing to account for how irrational members of
society, such as children, ought to be treated. Yet Mill's theory relies upon the proper upbringing of children.
Plank has asserted that Mill fails to account for physical harm, solely
concerning himself with spiritual wellbeing. He also argues that, while
much of Mill's theory depends upon a distinction between private and
public harm, Mill seems not to have provided a clear focus on or
distinction between the private and public realms.
Religious criticism
Nigel
Warburton states that though Mill encourages religious tolerance,
because he does not speak from the perspective of a specific religion,
some claim that he does not account for what certain religious beliefs
would entail when governing a society. Some religions believe that they
have a God given duty to enforce religious norms. For them, it seems
impossible for their religious beliefs to be wrong, i.e. the beliefs are
infallible. Therefore, according to Warburton, Mill's principle of
total freedom of speech may not apply.
Conception of harm
The harm principle is central to the principles in On Liberty.
Nigel Warburton says that Mill appears unclear about what constitutes
harm. Early in the book, he claims that simply being offensive does not
constitute harm. Later, he writes that certain acts which are permissible and harmless in private are worthy of being prohibited in public.
This seems to contradict his earlier claim that merely offensive acts
do not warrant prohibition because, presumably, the only harm done by a
public act which is harmless in private is that it is offensive.
Warburton notes that some people argue that morality is the basis
of society, and that society is the basis of individual happiness.
Therefore, if morality is undermined, so is individual happiness. Hence,
since Mill claims that governments ought to protect the individual's
ability to seek happiness, governments ought to intervene in the private
realm to enforce moral codes.
Charges of racism and colonialism
Mill
is clear that his concern for liberty does not extend to all
individuals and all societies. He states that "Despotism is a legitimate
mode of government in dealing with barbarians". Contemporary philosophers Domenico Losurdo and David Theo Goldberg have strongly criticised Mill as a racist and an apologist for colonialism. However, during his term as a Member of Parliament, he chaired the extraparliamentary Jamaica Committee, which for two years unsuccessfully sought the prosecution of Governor Eyre and his subordinates for military violence against Jamaican Blacks.
Diagram of factors that determine climate sensitivity. After increasing CO 2 levels, there is an initial warming. This warming gets amplified by the net effect of feedbacks. Self-reinforcing feedbacks include the melting of sunlight-reflecting ice, and higher evaporation increasing average atmospheric water vapour (a greenhouse gas).
Climate sensitivity is a measure of how much the Earth's
climate will cool or warm after a change in the climate system, for
instance, how much it will warm for doubling in carbon dioxide (CO 2) concentrations. In technical terms, climate sensitivity is the average change in the Earth's surface temperature in response to changes in radiative forcing, the difference between incoming and outgoing energy on Earth. Climate sensitivity is a key measure in climate science, and a focus area for climate scientists, who want to understand the ultimate consequences of anthroprogenic climate change.
The Earth's surface warms as a direct consequence of increased atmospheric CO 2, as well as increased concentrations of other greenhouse gases such as nitrous oxide and methane. Increasing temperatures have secondary effects on the climate system, such as an increase in atmospheric water vapour, which is itself also a greenhouse gas. Because scientists do not know exactly how strong these climate feedbacks
are, it is difficult to precisely predict the amount of warming that
will result from a given increase in greenhouse gas concentrations. If
climate sensitivity turns out to be on the high side of scientific
estimates, the Paris Agreement goal of limiting global warming to below 2 °C (3.6 °F) will be difficult to achieve.
The two primary types of climate sensitivity are the shorter-term
"transient climate response", the increase in global average
temperature that is expected to have occurred at a time when the
atmospheric CO 2
concentration has doubled; and "equilibrium climate sensitivity", the
higher long-term increase in global average temperature expected to
occur after the effects of a doubled CO 2
concentration have had time to reach a steady state. Climate
sensitivity is typically estimated in three ways; using direct
observations of temperature and levels of greenhouse gases taken during
the industrial age; using indirectly estimated temperature and other measurements from the Earth's more distant past; and modelling the various aspects of the climate system with computers.
Background
The rate at which energy reaches Earth as sunlight, and leaves Earth as heat radiation to space, must balance,
or the total amount of heat energy on the planet at any one time will
rise or fall, resulting in a planet that is warmer or cooler overall. An
imbalance between the rates of incoming and outgoing radiation energy
is called radiative forcing. A warmer planet radiates heat to space faster, so eventually a new balance is reached, with a higher planetary temperature. However, the warming of the planet also has knock-on effects. These knock-on effects create further warming, in an exacerbating feedback loop. Climate sensitivity is a measure of how much temperature change a given amount of radiative forcing will cause.
Radiative forcing
Radiative forcing is generally defined as the imbalance between incoming and outgoing radiation at the top of the atmosphere. Radiative forcing is measured in Watts per square meter (W/m2), the average imbalance in energy per second for each square meter of the Earth's surface.
Changes to radiative forcing lead to long-term changes in global temperature. A number of factors can affect radiative forcing: increased downwelling radiation due to the greenhouse effect, variability in solar radiation due to changes in planetary orbit, changes in solar irradiance, direct and indirect effects caused by aerosols (for example changes in albedo due to cloud cover), and changes in land use (i.e. deforestation or loss of reflective ice cover). In contemporary research, radiative forcing by greenhouse gases is well understood. As of 2019, large uncertainties remain for aerosols.
Key numbers
Carbon dioxide (CO 2) levels rose from 280 parts per million (ppm) in the eighteenth century, when humans in the Industrial Revolution started burning significant amounts of fossil fuel such as coal, to over 415 ppm by 2020. As CO 2 is a greenhouse gas, it hinders heat energy from leaving the Earth's atmosphere. In 2016, atmospheric CO 2 levels had increased by 45% over preindustrial levels, and radiative forcing caused by increased CO 2 was already more than 50% higher than in preindustrial times (due to non-linear effects).
Between the start of the Industrial Revolution in the eighteenth
century and 2020, the Earth's temperature rose by a little over one
degree Celsius (about two degrees Fahrenheit).
Societal importance
Because the economics of climate change mitigation depend a lot on how quickly carbon neutrality
needs to be achieved, climate sensitivity estimates can have important
economic and policy-making implications. One study suggests that halving
the uncertainty of the value for transient climate response (TCR) could
save trillions of dollars.
Scientists are uncertain about the precision of estimates of greenhouse
gas increases on future temperature – a higher climate sensitivity
would mean more dramatic increases in temperature – which makes it more
prudent to take significant climate action. If climate sensitivity turns out to be on the high end of what scientists estimate, it will be impossible to achieve the Paris Agreement
goal of limiting global warming to well below 2 °C; temperature
increases will exceed that limit, at least temporarily. One study
estimated that emissions cannot be reduced fast enough to meet the 2 °C
goal if equilibrium climate sensitivity (the long-term measure) is
higher than 3.4 °C (6.1 °F).
The more sensitive the climate system is to changes in greenhouse gas
concentrations, the more likely it is to have decades when temperatures
are much higher or much lower than the longer-term average.
Contributors to climate sensitivity
Radiative forcing is one component of climate sensitivity. The radiative forcing caused by a doubling of atmospheric CO 2 levels (from the preindustrial 280 ppm) is approximately 3.7 watts per square meter (W/m2). In the absence of feedbacks, this energy imbalance would eventually result in roughly 1 °C (1.8 °F) of global warming. This figure is straightforward to calculate using the Stefan-Boltzmann law and is undisputed.
A further contribution arises from climate feedbacks, both exacerbating and suppressing. The uncertainty in climate sensitivity estimates is due entirely to modeling of feedbacks in the climate system, including water vapour feedback, ice-albedo feedback, cloud feedback, and lapse rate feedback.
Suppressing feedbacks tend to counteract warming, increasing the rate
at which energy is radiated to space from a warmer planet. Exacerbating
feedbacks increase warming; for example, higher temperatures can cause
ice to melt, reducing the ice area and the amount of sunlight the ice
reflects, resulting in less heat energy being radiated back into space.
Climate sensitivity depends on the balance between these feedbacks.
Measures of climate sensitivity
Schematic of how different measures of climate sensitivity relate to one another
Depending on the time scale, there are two main ways to define climate sensitivity: the short-term transient climate response (TCR) and the long-term equilibrium climate sensitivity (ECS), which both incorporate the warming from exacerbating feedback loops. These are not discrete categories; they overlap. Sensitivity to atmospheric CO 2 increases is measured in the amount of temperature change for doubling in the atmospheric CO 2 concentration.
Although "climate sensitivity" is usually used for the sensitivity to radiative forcing caused by rising atmospheric CO 2, it is a general property of the climate system. Other agents can also cause a radiative imbalance. Climate sensitivity is the change in surface air temperature per unit change in radiative forcing, and the climate sensitivity parameter is therefore expressed in units of °C/(W/m2). Climate sensitivity is approximately the same, whatever the reason for the radiative forcing (e.g. from greenhouse gases or solar variation). When climate sensitivity is expressed as the temperature change for a level of atmospheric CO 2 double the pre-industrial level, its units are degrees Celsius (°C).
Transient climate response
The
transient climate response (TCR) is defined as "is the change in the
global mean surface temperature, averaged over a 20-year period,
centered at the time of atmospheric carbon dioxide doubling, in a
climate model simulation" in which the atmospheric CO 2 concentration is increasing at 1% per year. This estimate is generated using shorter-term simulations.
The transient response is lower than the equilibrium climate
sensitivity, because slower feedbacks, which exacerbate the temperature
increase, take more time to respond in full to an increase in the
atmospheric CO 2
concentration. For instance, the deep ocean takes many centuries to
reach a new steady state after a perturbation; during this time, it
continues to serve as heatsink, cooling the upper ocean. The IPCC literature assessment estimates that TCR likely lies between 1 °C (1.8 °F) and 2.5 °C (4.5 °F).
A related measure is the transient climate response to cumulative carbon emissions (TCRE), which is the globally averaged surface temperature change after 1000 GtC of CO 2 has been emitted. As such, it includes not only temperature feedbacks to forcing, but also the carbon cycle and carbon cycle feedbacks.
Equilibrium climate sensitivity
The equilibrium climate sensitivity (ECS) is the long-term temperature rise (equilibrium global mean near-surface air temperature) that is expected to result from a doubling of the atmospheric CO 2 concentration (ΔT2×). It is a prediction of the new global mean near-surface air temperature once the CO 2
concentration has stopped increasing and most of the feedbacks have had
time to have their full effect. Reaching an equilibrium temperature can
take centuries, or even millennia, after CO 2 has doubled. ECS is higher than TCR due to the oceans' short-term buffering effects. Computer models are used to estimate ECS.
A comprehensive estimate means modelling the whole time span during
which significant feedbacks continue to change global temperatures in
the model; for instance, fully equilibrating ocean temperatures requires
running a computer model that covers thousands of years. There are,
however, less computing-intensive methods.
The IPCC Fifth Assessment Report (AR5)
stated that "there is high confidence that ECS is extremely unlikely to
be less than 1 °C and medium confidence that the ECS is likely between
1.5 °C and 4.5 °C and very unlikely greater than 6 °C". The long time scales involved with ECS make it arguably a less relevant measure for policy decisions around climate change.
Effective climate sensitivity
A common approximation to ECS is the effective
equilibrium climate sensitivity. The effective climate sensitivity is
an estimate of equilibrium climate sensitivity using data from a climate
system, either in a model or real-world observations, that is not yet
in equilibrium.
Estimates assume that the net amplification effect of feedbacks (as
measured after some period of warming) will remain constant afterwards. This is not necessarily true, as feedbacks can change with time.
In many climate models, feedbacks become stronger over time, so that
the effective climate sensitivity is lower than the real ECS.
Earth system sensitivity
By
definition, equilibrium climate sensitivity does not includes feedbacks
that take millennia to emerge, such as long-term changes in Earth's
albedo due to changes in ice sheets and vegetation. It does include the
slow response of the deep ocean warming up, which also takes millennia,
and as such ECS doesn't reflect the actual future warming that would
occur if CO 2 is stabilized at double pre-industrial values. Earth system sensitivity (ESS) incorporates the effects of these slower feedback loops, such as the change in Earth's albedo from the melting of large continental ice sheets (which covered much of the northern hemisphere during the Last Glacial Maximum, and currently cover Greenland and Antarctica). Changes in albedo as a result of vegetation changes, and changes in ocean circulations are also included. These longer-term feedback loops make the ESS larger than the ECS – possibly twice as large. Data from Earth's geological history
is used to estimate ESS. Differences between modern and long-past
climatic conditions mean that estimates of future ESS are highly
uncertain. Like for ECS and TCR, the carbon cycle is not included in the definition of ESS, but all other elements of the climate system are.
Sensitivity to nature of the forcing
Different
forcing agents, such as greenhouse gases and aerosols, can be compared
using their radiative forcing (which is the initial radiative imbalance
averaged over the entire globe). Climate sensitivity is the amount of
warming per radiative forcing. To a first approximation, it does not
matter the cause of the radiative imbalance is, whether it is greenhouse
gases or something else. However, radiative forcing from sources other
than CO 2 can cause a somewhat larger or smaller surface warming than a similar radiative forcing due to CO 2; the amount of feedback varies, mainly because these forcings are not uniformly distributed over the globe. Forcings that initially warm the northern hemisphere, land, or polar regions more strongly are systematically more effective at changing temperatures than an equivalent forcing due to CO 2,
whose forcing is more uniformly distributed over the globe. This is
because these regions have more self-reinforcing feedbacks, such as the
ice-albedo feedback. Several studies indicate that human-emitted
aerosols are more effective than CO 2 at changing global temperatures, while volcanic forcing is less effective. When climate sensitivity to CO 2
forcing is estimated using historical temperature and forcing (caused
by a mix of aerosols and greenhouse gases), and this effect is not taken
into account, climate sensitivity will be underestimated.
State dependence
Artist impression of a snowball Earth state.
While climate sensitivity has been defined as the short- or long-term temperature change resulting from any doubling of CO 2, there is evidence that the sensitivity of Earth's climate system is not constant. For instance, the planet has polar ice and high-altitude glaciers. Until the world's ice has completely melted, an exacerbating ice-albedo feedback loop makes the system more sensitive overall.
Throughout Earth's history, there are thought to have been multiple
periods where snow and ice covered almost the entire globe. In most
models of this "snowball Earth" state, parts of the tropics were at
least intermittently free of ice cover. As the ice was advancing or
retreating, climate sensitivity would have been very high, as the large
changes in area of ice cover would have made for a very strong ice-albedo feedback.
Volcanic atmospheric composition changes are thought to have provided
the radiative forcing needed to escape the snowball state.
Equilibrium climate sensitivity can change with climate.
Climate sensitivity may further change if tipping points
are crossed. It is unlikely that tipping points will cause short-term
changes in climate sensitivity. If a tipping point is crossed, climate
sensitivity is expected to change at the time scale of the subsystem
that is hitting its tipping point. Especially if there are multiple
interacting tipping points, the transition of climate to a new state may
be difficult to reverse.
The two most used definitions of climate sensitivity specify the
climate state: ECS and TCR are defined for a doubling with respect to
the CO 2
levels in the pre-industrial era. Because of potential changes in
climate sensitivity, the climate system may warm by a different amount
after a second doubling of CO 2
than after a first doubling. The effect of any change in climate
sensitivity is expected to be small or negligible in the first century
after additional CO 2 is released into the atmosphere.
Estimating climate sensitivity
Historical estimates
Svante Arrhenius, in the 19th century, was the first person to quantify global warming as a consequence of a doubling of CO 2
concentration. In his first paper on the matter, he estimated that
global temperature would rise by around 5 to 6 °C (9.0 to 10.8 °F) if
the quantity of CO 2 was doubled. In later work, he revised this estimate to 4 °C (7.2 °F). Arrhenius used Samuel Pierpont Langley's observations of radiation emitted by the full moon to estimate the amount of radiation that was absorbed by water vapour and CO 2. To account for water vapour feedback, he assumed that relative humidity would stay the same under global warming.
The first calculation of climate sensitivity using detailed measurements of absorption spectra, and the first to use a computer to numerically integrate the radiative transfer through the atmosphere, was performed by Syukuro Manabe and Richard Wetherald in 1967. Assuming constant humidity, they computed an equilibrium climate sensitivity of 2.3 °C per doubling of CO 2
(which they rounded to 2 °C, the value most often quoted from their
work, in the abstract of the paper). This work has been called "arguably
the greatest climate-science paper of all time" and "the most influential study of climate of all time."
A committee on anthropogenic global warming, convened in 1979 by the United States National Academy of Sciences and chaired by Jule Charney,
estimated equilibrium climate sensitivity to be 3 °C (5.4 °F), plus or
minus 1.5 °C (2.7 °F). The Manabe and Wetherald estimate (2 °C
(3.6 °F)), James E. Hansen's
estimate of 4 °C (7.2 °F), and Charney's model were the only models
available in 1979. According to Manabe, speaking in 2004, "Charney chose
0.5 °C as a reasonable margin of error, subtracted it from Manabe's
number, and added it to Hansen's, giving rise to the 1.5 to 4.5 °C (2.7
to 8.1 °F) range of likely climate sensitivity that has appeared in
every greenhouse assessment since ...." In 2008, climatologist Stefan Rahmstorf
said: "At that time [it was published], the [Charney report estimate's]
range [of uncertainty] was on very shaky ground. Since then, many
vastly improved models have been developed by a number of climate
research centers around the world."
Intergovernmental Panel on Climate Change
Historical
estimates of climate sensitivity from the IPCC assessments. The first
three reports gave a qualitative likely range, while the fourth and
fifth assessment report formally quantified the uncertainty. The dark
blue range is judged as being more than 66% likely.
Despite considerable progress in the understanding of Earth's climate system,
assessments continued to report similar uncertainty ranges for climate
sensitivity for some time after the 1979 Charney report. The 1990 IPCC First Assessment Report estimated that equilibrium climate sensitivity to a doubling of CO 2 lay between 1.5 and 4.5 °C (2.7 and 8.1 °F), with a "best guess in the light of current knowledge" of 2.5 °C (4.5 °F). This report used models with simplified representations of ocean dynamics. The IPCC supplementary report, 1992, which used full-ocean circulation models, saw "no compelling reason to warrant changing" the 1990 estimate; and the IPCC Second Assessment Report said that "No strong reasons have emerged to change [these estimates]".
In these reports, much of the uncertainty around climate sensitivity
was attributed to insufficient knowledge of cloud processes. The 2001 IPCC Third Assessment Report also retained this likely range.
Authors of the 2007 IPCC Fourth Assessment Report
stated that confidence in estimates of equilibrium climate sensitivity
had increased substantially since the Third Annual Report.
The IPCC authors concluded that ECS is very likely to be greater than
1.5 °C (2.7 °F) and likely to lie in the range 2 to 4.5 °C (3.6 to
8.1 °F), with a most likely value of about 3 °C (5.4 °F). The IPCC
stated that, due to fundamental physical reasons and data limitations, a
climate sensitivity higher than 4.5 °C (8.1 °F) could not be ruled out,
but that the climate sensitivity estimates in the likely range agreed
better with observations and proxy climate data.
The 2013 IPCC Fifth Assessment Report
reverted to the earlier range of 1.5 to 4.5 °C (2.7 to 8.1 °F) (with
high confidence), because some estimates using industrial-age data came
out low. (See the next section for details.)
The report also stated that ECS is extremely unlikely to be less than
1 °C (1.8 °F) (high confidence), and is very unlikely to be greater than
6 °C (11 °F) (medium confidence). These values were estimated by
combining the available data with expert judgement.
When the Ipcc begun to produce its IPCC Sixth Assessment Report many climate models begun to show higher climate sensitivity. The estimates for Equilibrium Climate Sensitivity changed from 3.2 °C to 3.7 °C and the estimates for the Transient climate response from 1.8 °C, to 2.0 °C. This is probably due to better understanding of the role of clouds and aerosols.
Methods of estimation
Using industrial-age (1750–present) data
Climate
sensitivity can be estimated using observed temperature rise, observed
ocean heat uptake, and modelled or observed radiative forcing. These
data are linked though a simple energy-balance model to calculate
climate sensitivity. Radiative forcing is often modelled, because Earth observation satellites
that measure it existed during only part of the industrial age (only
since the mid-20th century). Estimates of climate sensitivity calculated
using these global energy constraints have consistently been lower than
those calculated using other methods, around 2 °C (3.6 °F) or lower.
Estimates of transient climate response (TCR) calculated from
models and observational data can be reconciled if it is taken into
account that fewer temperature measurements are taken in the polar
regions, which warm more quickly than the Earth as a whole.
If only regions for which measurements are available are used in
evaluating the model, differences in TCR estimates are negligible.
A very simple climate model could estimate climate sensitivity from industrial-age data by waiting for the climate system to reach equilibrium and then measuring the resulting warming, ΔTeq (°C). Computation of the equilibrium climate sensitivity, S (°C), using the radiative forcing ΔF (W/m2) and the measured temperature rise, would then be possible. The radiative forcing resulting from a doubling of CO 2, F2CO2, is relatively well known, at about 3.7 W/m2. Combining this information results in the following equation:
.
However, the climate system is not in equilibrium. Actual warming
lags the equilibrium warming, largely because the oceans take up heat
and will take centuries or millennia to reach equilibrium.
Estimating climate sensitivity from industrial-age data requires an
adjustment to the equation above. The actual forcing felt by the
atmosphere is the radiative forcing minus the ocean's heat uptake, H (W/m2), so that climate sensitivity can be estimated by:
The global temperature increase between the beginning of the industrial period (taken as 1750) and 2011 was about 0.85 °C (1.53 °F). In 2011, the radiative forcing due to CO 2 and other long-lived greenhouse gases – mainly methane, nitrous oxide, and chlorofluorocarbons – emitted since the eighteenth century was roughly 2.8 W/m2. The climate forcing, ΔF, also contains contributions from solar activity (+0.05 W/m2), aerosols (−0.9 W/m2), ozone (+0.35 W/m2), and other smaller influences, bringing the total forcing over the industrial period to 2.2 W/m2, according to the best estimate of the IPCC AR5, with substantial uncertainty. The ocean heat uptake estimated by the IPCC AR5 as 0.42 W/m2, yields a value for S of 1.8 °C (3.2 °F).
Other strategies
In
theory, industrial-age temperatures could also be used to determine a
timescale for the temperature response of the climate system, and thus
climate sensitivity: if the effective heat capacity of the climate system is known, and the timescale is estimated using autocorrelation
of the measured temperature, an estimate of climate sensitivity can be
derived. In practice, however, simultaneous determination of the
timescale and heat capacity is difficult.
Attempts have been made to use the 11-year solar cycle to constrain the transient climate response. Solar irradiance is about 0.9 W/m2 higher during a solar maximum than during a solar minimum, and the effects of this can be observed in measured average global temperatures over the period 1959–2004. Unfortunately, the solar minima in this period coincided with volcanic eruptions, which have a cooling effect on the global temperature.
Because the eruptions caused a larger and less well quantified decrease
in radiative forcing than the reduced solar irradiance, it is
questionable whether useful quantitative conclusions can be derived from
the observed temperature variations.
Observations of volcanic eruptions have also been used to try to
estimate climate sensitivity, but as the aerosols from a single eruption
last at most a couple of years in the atmosphere, the climate system
can never come close to equilibrium, and there is less cooling than
there would be if the aerosols stayed in the atmosphere for longer.
Therefore, volcanic eruptions give information only about a lower bound on transient climate sensitivity.
Using data from Earth's past
Historical climate sensitivity can be estimated by using reconstructions of Earth's past temperatures and CO 2 levels. Paleoclimatologists have studied different geological periods, such as the warm Pliocene (5.3 to 2.6 million years ago) and the colder Pleistocene (2.6 million to 11,700 years ago),
seeking periods that are in some way analogous to or informative about
current climate change. Climates further back in Earth's history are
more difficult to study, because less data is available about them. For
instance, past CO 2 concentrations can be derived from air trapped in ice cores, but as of 2020, the oldest continuous ice core is less than one million years old. Recent periods, such as the Last Glacial Maximum (LGM) (about 21,000 years ago) and the Mid-holocene (about 6,000 years ago), are often studied, especially when more information about them becomes available.
A 2007 estimate of sensitivity made using data from the most
recent 420 million years is consistent with sensitivities of current
climate models and with other determinations. The Paleocene–Eocene Thermal Maximum
(about 55.5 million years ago), a 20,000-year period during which
massive amount of carbon entered the atmosphere and average global
temperatures increased by approximately 6 °C (11 °F), also provides a
good opportunity to study the climate system when it was in a warm
state. Studies of the last 800,000 years have concluded that climate sensitivity was greater in glacial periods than in interglacial periods.
As the name suggests, the LGM was a lot colder than today; there is good data on atmospheric CO 2 concentrations and radiative forcing during that period. While orbital forcing was different from that of the present, it had little effect on mean annual temperatures. Estimating climate sensitivity from the LGM can be done in several different ways.
One way is to use estimates of global radiative forcing and temperature
directly. The set of feedback mechanisms active during the LGM,
however, may be different from the feedbacks caused by a doubling of CO 2 in the present, introducing additional uncertainty.
In a different approach, a model of intermediate complexity is used to
simulate conditions during the LGM. Several versions of this single
model are run, with different values chosen for uncertain parameters,
such that each version has a different ECS. Outcomes that best simulate
observed cooling during the LGM probably produce the most realistic ECS
values.
Using climate models
Frequency distribution of equilibrium climate sensitivity, based on simulations of doubling CO 2.
Each model simulation has different estimates for processes that
scientists do not sufficiently understand. Few of the simulations result
in less than 2 °C (3.6 °F) of warming or significantly more than 4 °C
(7.2 °F). However, the positive skew, which is also found in other studies,
suggests that if carbon dioxide concentrations double, the probability
of large or very large increases in temperature is greater than the
probability of small increases.
Climate models are used to simulate the CO 2-driven warming of the future as well as the past. They operate on principles similar to those underlying models that predict the weather,
but they focus on longer-term processes. Climate models typically begin
with a starting state, then apply physical laws and knowledge about
biology to generate subsequent states. As with weather modeling, no
computer has the power to model the full complexity of the entire
planet, so simplifications are used to reduce this complexity to
something manageable. An important simplification divides Earth's
atmosphere into model cells. For instance, the atmosphere might be
divided into cubes of air ten or one hundred kilometers on a side. Each
model cell is treated as if it were homogeneous. Calculations for model cells are much faster than trying to simulate each molecule of air separately.
A lower model resolution
(large model cells, long time steps) takes less computing power, but it
cannot simulate the atmosphere in as much detail. A model is unable
able to simulate processes smaller than the model cells or shorter-term
than a single time step. The effects of these smaller-scale (and
shorter-term) processes must therefore be estimated using other methods.
Physical laws contained in the models may also be simplified to speed
up calculations. The biosphere
must be included in climate models. The effects of the biosphere are
estimated using data on the average behaviour of the average plant
assemblage of an area under the modelled conditions. Climate sensitivity
is therefore an emergent property of these models; it is not prescribed, but follows from the interaction of all the modelled processes.
To estimate climate sensitivity, a model is run using a variety
of radiative forcings (doubling quickly, doubling gradually, or
following historical emissions) and the temperature results are compared
to the forcing applied. Different models give different estimates of
climate sensitivity, but they tend to fall within a similar range, as
described above.
Testing, comparisons, and estimates
Modelling
of the climate system can lead to a wide range of outcomes. Models are
often run using different plausible parameters in their approximation of
physical laws and the behaviour of the biosphere, forming a perturbed physics ensemble
that attempts to model the sensitivity of the climate to different
types and amounts of change in each parameter. Alternatively,
structurally different models developed at different institutions are
put together, creating an ensemble. By selecting only those simulations
that can simulate some part of the historical climate well, a
constrained estimate of climate sensitivity can be made. One strategy
for obtaining more accurate results is placing more emphasis on climate
models that perform well in general.
A model is tested using observations, paleoclimate data, or both
to see if it replicates them accurately. If it does not, inaccuracies in
the physical model and parametrizations are sought and the model is
modified. For models used to estimate climate sensitivity, specific test
metrics that are directly and physically linked to climate sensitivity
are sought; examples of such metrics are the global patterns of warming, the ability of a model to reproduce observed relative humidity in the tropics and subtropics, patterns of heat radiation, and the variability of temperature around long-term historical warming.
Ensemble climate models developed at different institutions tend to
produce constrained estimates of ECS that are slightly higher than 3 °C
(5.4 °F); the models with ECS slightly above 3 °C (5.4 °F) simulate the
above situations better than models with a lower climate sensitivity.
Many projects and groups exist which compare and analyse the results of multiple models. For instance, the Coupled Model Intercomparison Project (CMIP) has been running since the 1990s.
In preparation for the 2021 6th IPCC report, a new generation of climate models have been developed by scientific groups around the world.
The average estimated climate sensitivity has increased in Coupled
Model Intercomparison Project phase 6 (CMIP6) compared to the previous
generation, with values spanning 1.8 to 5.6 °C (3.2 to 10.1 °F) across
27 global climate models and exceeding 4.5 °C (8.1 °F) in 10 of them.
The cause of the increased ECS lies mainly in improved modelling of
clouds; temperature rises are now believed to cause sharper decreases in
the number of low clouds, and fewer low clouds means more sunlight is
absorbed by the planet rather than reflected back into space. Models with the highest ECS values, however, are not consistent with observed warming.
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