Carbon sequestration

From Wikipedia, the free encyclopedia

Schematic showing both terrestrial and geological sequestration of carbon dioxide emissions from a coal-fired plant.

 

Carbon sequestration is the process involved in carbon capture and the long-term storage of atmospheric carbon dioxide or other forms of carbon to mitigate or defer global warming. It has been proposed as a way to slow the atmospheric and marine accumulation of greenhouse gases, which are released by burning fossil fuels.

Carbon dioxide (CO
₂) is naturally captured from the atmosphere through biological, chemical, and physical processes. Artificial processes have been devised to produce similar effects, including large-scale, artificial capture and sequestration of industrially produced CO
₂ using subsurface saline aquifers, reservoirs, ocean water, aging oil fields, or other carbon sinks.

Description

Carbon sequestration is the process involved in carbon capture and the long-term storage of atmospheric carbon dioxide (CO
₂) and may refer specifically to:

• "The process of removing carbon from the atmosphere and depositing it in a reservoir." When carried out deliberately, this may also be referred to as carbon dioxide removal, which is a form of geoengineering.
• Carbon capture and storage, where carbon dioxide is removed from flue gases (e.g., at power stations) before being stored in underground reservoirs.
• Natural biogeochemical cycling of carbon between the atmosphere and reservoirs, such as by chemical weathering of rocks.

Carbon dioxide may be captured as a pure by-product in processes related to petroleum refining or from flue gases from power generation. CO
₂ sequestration includes the storage part of carbon capture and storage, which refers to large-scale, artificial capture and sequestration of industrially produced CO
₂ using subsurface saline aquifers, reservoirs, ocean water, aging oil fields, or other carbon sinks.

Carbon sequestration describes long-term storage of carbon dioxide or other forms of carbon to either mitigate or defer global warming and avoid dangerous climate change. It has been proposed as a way to slow the atmospheric and marine accumulation of greenhouse gases, which are released by burning fossil fuels.

Carbon dioxide is naturally captured from the atmosphere through biological, chemical or physical processes. Some artificial sequestration techniques exploit these natural processes, while some use entirely artificial processes. 

There are three ways that this sequestration can be carried out; post-combustion capture, pre-combustion capture, and oxy-combustion. A wide variety of separation techniques are being pursued, including gas phase separation, absorption into a liquid, and adsorption on a solid, as well as hybrid processes, such as adsorption/membrane systems. These above processes basically will capture carbon emitting from power plants, factories, fuel burning industries and so on.

Biological processes

An oceanic phytoplankton bloom in the South Atlantic Ocean, off the coast of Argentina. Encouraging such blooms with iron fertilization could lock up carbon on the seabed.

 

Biosequestration or carbon sequestration through biological processes affects the global carbon cycle. Examples include major climatic fluctuations, such as the Azolla event, which created the current Arctic climate. Such processes created fossil fuels, as well as clathrate and limestone. By manipulating such processes, geoengineers seek to enhance sequestration.

Peat production

Peat bogs act as a sink for carbon due to the accumulation of partially decayed biomass that would otherwise continue to decay completely. There is a variance on how much the peatlands act as a carbon sink or carbon source that can be linked to varying climates in different areas of the world and different times of the year. By creating new bogs, or enhancing existing ones, the amount of carbon that is sequestered by bogs would increase.

Forestry

Reforestation is the replanting of trees on marginal crop and pasture lands to incorporate carbon from atmospheric CO
₂ into biomass. For this process to succeed the carbon must not return to the atmosphere from mass burning or rotting when the trees die. To this end, land allotted to the trees must not be converted to other uses and management of the frequency of disturbances might be necessary in order to avoid extreme events. Alternatively, the wood from them must itself be sequestered, e.g., via biochar, bio-energy with carbon storage (BECS), landfill or 'stored' by use in e.g. construction. Short of growth in perpetuity, however, reforestation with long-lived trees (>100 years) will sequester carbon for a more graduated release, minimizing impact during the expected carbon crisis of the 21st century.

Urban forestry

Urban forestry increases the amount of carbon taken up in cities by adding new tree sites and the sequestration of carbon occurs over the lifetime of the tree. It is generally practiced and maintained on smaller scales, like in cities. The results of urban forestry can have different results depending on the type of vegetation that is being used, so it can function as a sink but can also function as a source of emissions. Along with sequestration by the plants which is difficult to measure but seems to have little effect on the overall amount of carbon dioxide that is uptaken, the vegetation can have indirect effects on carbon by reducing need for energy consumption.

Wetland restoration

Wetland soil is an important carbon sink; 14.5% of the world's soil carbon is found in wetlands, while only 6% of the world's land is composed of wetlands.

Agriculture

Compared to natural vegetation, cropland soils are depleted in soil organic carbon (SOC). When a soil is converted from natural land or semi natural land, such as forests, woodlands, grasslands, steppes and savannas, the SOC content in the soil reduces with about 30–40%. This loss is due to the removal of plant material containing carbon, in terms of harvests. When the land use changes, the carbon in the soil will either increase or decrease, this change will continue until the soil reaches a new equilibrium. Deviations from this equilibrium can also be affected by variated climate. The decreasing of SOC content can be counteracted by increasing the carbon input, this can be done with several strategies, e.g. leave harvest residues on the field, use manure as fertilizer or include perennial crops in the rotation. Perennial crops have larger below ground biomass fraction, which increases the SOC content. Globally, soils are estimated to contain approximately 1,500 gigatons of organic carbon to 1 m depth, more than the amount in vegetation and the atmosphere.

Modification of agricultural practices is a recognized method of carbon sequestration as soil can act as an effective carbon sink offsetting as much as 20% of 2010 carbon dioxide emissions annually.

Carbon emission reduction methods in agriculture can be grouped into two categories: reducing and/or displacing emissions and enhancing carbon removal. Some of these reductions involve increasing the efficiency of farm operations (e.g. more fuel-efficient equipment) while some involve interruptions in the natural carbon cycle. Also, some effective techniques (such as the elimination of stubble burning) can negatively impact other environmental concerns (increased herbicide use to control weeds not destroyed by burning).

Deep soil

Soils hold four times the amount of carbon stored in the atmosphere. About half of this is found deep within soils. About 90% of this deep soil C is stabilized by mineral-organic associations.

Reducing emissions

Increasing yields and efficiency generally reduces emissions as well, since more food results from the same or less effort. Techniques include more accurate use of fertilizers, less soil disturbance, better irrigation, and crop strains bred for locally beneficial traits and increased yields.

Replacing more energy intensive farming operations can also reduce emissions. Reduced or no-till farming requires less machine use and burns correspondingly less fuel per acre. However, no-till usually increases use of weed-control chemicals and the residue now left on the soil surface is more likely to release its CO
₂ to the atmosphere as it decays, reducing the net carbon reduction.

In practice, most farming operations that incorporate post-harvest crop residues, wastes and byproducts back into the soil provide a carbon storage benefit. This is particularly the case for practices such as field burning of stubble – rather than releasing almost all of the stored CO
₂ to the atmosphere, tillage incorporates the biomass back into the soil.

Enhancing carbon removal

All crops absorb CO
₂ during growth and release it after harvest. The goal of agricultural carbon removal is to use the crop and its relation to the carbon cycle to permanently sequester carbon within the soil. This is done by selecting farming methods that return biomass to the soil and enhance the conditions in which the carbon within the plants will be reduced to its elemental nature and stored in a stable state. Methods for accomplishing this include:

• Use cover crops such as grasses and weeds as temporary cover between planting seasons
• Concentrate livestock in small paddocks for days at a time so they graze lightly but evenly. This encourages roots to grow deeper into the soil. Stock also till the soil with their hooves, grinding old grass and manures into the soil.
• Cover bare paddocks with hay or dead vegetation. This protects soil from the sun and allows the soil to hold more water and be more attractive to carbon-capturing microbes.
• Restore degraded land, which slows carbon release while returning the land to agriculture or other use.

Agricultural sequestration practices may have positive effects on soil, air, and water quality, be beneficial to wildlife, and expand food production. On degraded croplands, an increase of 1 ton of soil carbon pool may increase crop yield by 20 to 40 kilograms per hectare of wheat, 10 to 20 kg/ ha for maize, and 0.5 to 1 kg/ha for cowpeas.

The effects of soil sequestration can be reversed. If the soil is disrupted or tillage practices are abandoned, the soil becomes a net source of greenhouse gases. Typically after 15 to 30 years of sequestration, soil becomes saturated and ceases to absorb carbon. This implies that there is a global limit to the amount of carbon that soil can hold.

Many factors affect the costs of carbon sequestration including soil quality, transaction costs and various externalities such as leakage and unforeseen environmental damage. Because reduction of atmospheric CO
₂ is a long-term concern, farmers can be reluctant to adopt more expensive agricultural techniques when there is not a clear crop, soil, or economic benefit. Governments such as Australia and New Zealand are considering allowing farmers to sell carbon credits once they document that they have sufficiently increased soil carbon content.

Ocean-related

Iron fertilization

Ocean iron fertilization is an example of such a geoengineering technique. Iron fertilization attempts to encourage phytoplankton growth, which removes carbon from the atmosphere for at least a period of time. This technique is controversial due to limited understanding of its complete effects on the marine ecosystem, including side effects and possibly large deviations from expected behavior. Such effects potentially include release of nitrogen oxides, and disruption of the ocean's nutrient balance.

Natural iron fertilisation events (e.g., deposition of iron-rich dust into ocean waters) can enhance carbon sequestration. Sperm whales act as agents of iron fertilisation when they transport iron from the deep ocean to the surface during prey consumption and defecation. Sperm whales have been shown to increase the levels of primary production and carbon export to the deep ocean by depositing iron rich feces into surface waters of the Southern Ocean. The iron rich feces causes phytoplankton to grow and take up more carbon from the atmosphere. When the phytoplankton dies, some of it sinks to the deep ocean and takes the atmospheric carbon with it. By reducing the abundance of sperm whales in the Southern Ocean, whaling has resulted in an extra 200,000 tonnes of carbon remaining in the atmosphere each year.

Urea fertilization

Ian Jones proposes fertilizing the ocean with urea, a nitrogen rich substance, to encourage phytoplankton growth.

Australian company Ocean Nourishment Corporation (ONC) plans to sink hundreds of tonnes of urea into the ocean to boost CO
₂-absorbing phytoplankton growth as a way to combat climate change. In 2007, Sydney-based ONC completed an experiment involving 1 tonne of nitrogen in the Sulu Sea off the Philippines.

Mixing layers

Encouraging various ocean layers to mix can move nutrients and dissolved gases around, offering avenues for geoengineering. Mixing may be achieved by placing large vertical pipes in the oceans to pump nutrient rich water to the surface, triggering blooms of algae, which store carbon when they grow and export carbon when they die. This produces results somewhat similar to iron fertilization. One side-effect is a short-term rise in CO
₂, which limits its attractiveness.

Seaweed

Seaweed grows very fast and can theoretically be harvested and processed to generate biomethane, via Anaerobic Digestion to generate electricity, via Cogeneration/CHP or as a replacement for natural gas. One study suggested that if seaweed farms covered 9% of the ocean they could produce enough biomethane to supply Earth's equivalent demand for fossil fuel energy, remove 53 gigatons of CO₂ per year from the atmosphere and sustainably produce 200 kg per year of fish, per person, for 10 billion people. Ideal species for such farming and conversion include Laminaria digitata, Fucus serratus and Saccharina latissima.

Physical processes

Biochar can be landfilled, used as a soil improver or burned using carbon capture and storage

Biomass-related

Bio-energy with carbon capture and storage

Bio-energy with carbon capture and storage (BECCS) refers to biomass in power stations and boilers that use carbon capture and storage. The carbon sequestered by the biomass would be captured and stored, thus removing carbon dioxide from the atmosphere.

This technology is sometimes referred to as bio-energy with carbon storage, BECS, though this term can also refer to the carbon sequestration potential in other technologies, such as biochar.

Burial

Burying biomass (such as trees) directly, mimics the natural processes that created fossil fuels. Landfills also represent a physical method of sequestration.

Biochar burial

Biochar is charcoal created by pyrolysis of biomass waste. The resulting material is added to a landfill or used as a soil improver to create terra preta. Addition of pyrogenic organic carbon (biochar) is a novel strategy to increase the soil-C stock for the long-term and to mitigate global-warming by offsetting the atmospheric C (up to 9.5 Pg C annually).

In the soil, the carbon is unavailable for oxidation to CO
₂ and consequential atmospheric release. This is one technique advocated by scientist James Lovelock, creator of the Gaia hypothesis. According to Simon Shackley, "people are talking more about something in the range of one to two billion tonnes a year."

The mechanisms related to biochar are referred to as bio-energy with carbon storage, BECS.

Ocean storage

If CO₂ were to be injected to the ocean bottom, the pressures would be great enough for CO₂ to be in its liquid phase. The idea behind ocean injection would be to have stable, stationary pools of CO₂ at the ocean floor. The ocean could potentially hold over a thousand billion tons of CO₂. However, this avenue of sequestration isn't being as actively pursued because of concerns about the impact on ocean life, and concerns about its stability.

River mouths bring large quantities of nutrients and dead material from upriver into the ocean as part of the process that eventually produces fossil fuels. Transporting material such as crop waste out to sea and allowing it to sink exploits this idea to increase carbon storage. International regulations on marine dumping may restrict or prevent use of this technique.

Geological sequestration

Geological sequestration refers to the storage of CO₂ underground in depleted oil and gas reservoirs, saline formations, or deep, un-minable coal beds. 

Once CO₂ is captured from a gas or coal-fired power plant, it would be compressed to ≈100 bar so that it would be a supercritical fluid. In this fluid form, the CO₂ would be easy to transport via pipeline to the place of storage. The CO₂ would then be injected deep underground, typically around 1 km, where it would be stable for hundreds to millions of years. At these storage conditions, the density of supercritical CO₂ is 600 to 800 kg / m³. For consumers, the cost of electricity from a coal-fired power plant with carbon capture and storage (CCS) is estimated to be 0.01–0.05 $ / kWh higher than without CCS. For reference, the average cost of electricity in the US in 2004 was 0.0762 $ / kWh. In other terms, the cost of CCS would be 20–70 $/ton of CO₂ captured. The transportation and injection of CO₂ is relatively cheap, with the capture costs accounting for 70–80% of CCS costs.

The important parameters in determining a good site for carbon storage are: rock porosity, rock permeability, absence of faults, and geometry of rock layers. The medium in which the CO₂ is to be stored ideally has a high porosity and permeability, such as sandstone or limestone. Sandstone can have a permeability ranging from 1 to 10⁻⁵ Darcy, and can have a porosity as high as ≈30%. The porous rock must be capped by a layer of low permeability which acts as a seal, or caprock, for the CO₂. Shale is an example of a very good caprock, with a permeability of 10⁻⁵ to 10⁻⁹ Darcy. Once injected, the CO₂ plume will rise via buoyant forces, since it is less dense than its surroundings. Once it encounters a caprock, it will spread laterally until it encounters a gap. If there are fault planes near the injection zone, there is a possibility the CO₂ could migrate along the fault to the surface, leaking into the atmosphere, which would be potentially dangerous to life in the surrounding area. Another danger related to carbon sequestration is induced seismicity. If the injection of CO₂ creates pressures that are too high underground, the formation will fracture, causing an earthquake.

While trapped in a rock formation, CO₂ can be in the supercritical fluid phase or dissolve in groundwater/brine. It can also react with minerals in the geologic formation to precipitate carbonates. 

Worldwide storage capacity in oil and gas reservoirs is estimated to be 675–900 Gt CO₂, and in un-minable coal seams is estimated to be 15–200 Gt CO₂. Deep saline formations have the largest capacity, which is estimated to be 1,000–10,000 Gt CO₂. In the US, there is an estimated 160 Gt CO₂ storage capacity.

There are a number of large-scale carbon capture and sequestration projects that have demonstrated the viability and safety of this method of carbon storage, which are summarized here by the Global CCS Institute. The dominant monitoring technique is seismic imaging, where vibrations are generated that propagate through the subsurface. The geologic structure can be imaged from the refracted/reflected waves.

The first large-scale CO
₂ sequestration project which began in 1996 is called Sleipner, and is located in the North Sea where Norway's StatoilHydro strips carbon dioxide from natural gas with amine solvents and disposed of this carbon dioxide in a deep saline aquifer. In 2000, a coal-fueled synthetic natural gas plant in Beulah, North Dakota, became the world's first coal-using plant to capture and store carbon dioxide, at the Weyburn-Midale Carbon Dioxide Project.

CO
₂ has been used extensively in enhanced crude oil recovery operations in the United States beginning in 1972. There are in excess of 10,000 wells that inject CO
₂ in the state of Texas alone. The gas comes in part from anthropogenic sources, but is principally from large naturally occurring geologic formations of CO
₂. It is transported to the oil-producing fields through a large network of over 5,000 kilometres (3,100 mi) of CO
₂ pipelines. The use of CO
₂ for enhanced oil recovery (EOR) methods in heavy oil reservoirs in the Western Canadian Sedimentary Basin (WCSB) has also been proposed. However, transport cost remains an important hurdle. An extensive CO
₂ pipeline system does not yet exist in the WCSB. Athabasca oil sands mining that produces CO
₂ is hundreds of kilometers north of the subsurface Heavy crude oil reservoirs that could most benefit from CO
₂ injection.

Chemical processes

Developed in the Netherlands, an electrocatalysis by a copper complex helps reduce carbon dioxide to oxalic acid; This conversion uses carbon dioxide as a feedstock to generate oxalic acid.

Mineral carbonation

Carbon, in the form of CO
₂ can be removed from the atmosphere by chemical processes, and stored in stable carbonate mineral forms. This process is known as 'carbon sequestration by mineral carbonation' or mineral sequestration. The process involves reacting carbon dioxide with abundantly available metal oxides–either magnesium oxide (MgO) or calcium oxide (CaO)–to form stable carbonates. These reactions are exothermic and occur naturally (e.g., the weathering of rock over geologic time periods).

CaO + CO
₂ → CaCO
₃

MgO + CO
₂ → MgCO
₃

Calcium and magnesium are found in nature typically as calcium and magnesium silicates (such as forsterite and serpentinite) and not as binary oxides. For forsterite and serpentine the reactions are:

Mg
₂SiO
₄ + 2 CO
₂ → 2 MgCO
₃ + SiO
₂

Mg
₃Si
₂O
₅(OH)
₄+ 3 CO
₂ → 3 MgCO
₃ + 2 SiO
₂ + 2 H
₂O

The following table lists principal metal oxides of Earth's crust. Theoretically up to 22% of this mineral mass is able to form carbonates. 

Earthen Oxide	Percent of Crust	Carbonate	Enthalpy change (kJ/mol)
SiO 2	59.71
Al 2O 3	15.41
CaO	4.90	CaCO 3	−179
MgO	4.36	MgCO 3	−117
Na 2O	3.55	Na 2CO 3
FeO	3.52	FeCO 3
K 2O	2.80	K 2CO 3
Fe 2O 3	2.63	FeCO 3
	21.76	All Carbonates

These reactions are slightly more favorable at low temperatures. This process occurs naturally over geologic time frames and is responsible for much of the Earth's surface limestone. The reaction rate can be made faster however, by reacting at higher temperatures and/or pressures, although this method requires some additional energy. Alternatively, the mineral could be milled to increase its surface area, and exposed to water and constant abrasion to remove the inert Silica as could be achieved naturally by dumping Olivine in the high energy surf of beaches. Experiments suggest the weathering process is reasonably quick (one year) given porous basaltic rocks.

CO
₂ naturally reacts with peridotite rock in surface exposures of ophiolites, notably in Oman. It has been suggested that this process can be enhanced to carry out natural mineralisation of CO
₂.

When CO
₂ is dissolved in water and injected into hot basaltic rocks underground it has been shown that the CO
₂ reacts with the basalt to form solid carbonate minerals. A test plant in Iceland started up in October 2017, extracting up to 50 tons of CO2 a year from the atmosphere and storing it underground in basaltic rock.

Researchers from British Columbia, developed a low cost process for the production of magnesite, also known as magnesium carbonate, which can sequester CO2 from the air, or at the point of air pollution, e.g. at a power plant. The crystals are naturally occurring, but accumulation is usually very slow.

Industrial use

Traditional cement manufacture releases large amounts of carbon dioxide, but newly developed cement types from Novacem can absorb CO
₂ from ambient air during hardening. A similar technique was pioneered by TecEco, which has been producing "EcoCement" since 2002. A Canadian startup CarbonCure takes captured CO2 and injects it into concrete as it's being mixed. Carbon Upcycling UCLA is another company that uses CO
₂ in concrete. Their concrete product is called CO2NCRETE™, a concrete that hardens faster and is more eco-friendly than traditional concrete.

In Estonia, oil shale ash, generated by power stations could be used as sorbents for CO
₂ mineral sequestration. The amount of CO
₂ captured averaged 60 to 65% of the carbonaceous CO
₂ and 10 to 11% of the total CO
₂ emissions.

Chemical scrubbers

Various carbon dioxide scrubbing processes have been proposed to remove CO
₂ from the air, usually using a variant of the Kraft process. Carbon dioxide scrubbing variants exist based on potassium carbonate, which can be used to create liquid fuels, or on sodium hydroxide. These notably include artificial trees proposed by Klaus Lackner to remove carbon dioxide from the atmosphere using chemical scrubbers.

Ocean-related

Basalt storage

Carbon dioxide sequestration in basalt involves the injecting of CO
₂ into deep-sea formations. The CO
₂ first mixes with seawater and then reacts with the basalt, both of which are alkaline-rich elements. This reaction results in the release of Ca²⁺ and Mg²⁺ ions forming stable carbonate minerals.

Underwater basalt offers a good alternative to other forms of oceanic carbon storage because it has a number of trapping measures to ensure added protection against leakage. These measures include “geochemical, sediment, gravitational and hydrate formation.” Because CO
₂ hydrate is denser than CO
₂ in seawater, the risk of leakage is minimal. Injecting the CO
₂ at depths greater than 2,700 meters (8,900 ft) ensures that the CO
₂ has a greater density than seawater, causing it to sink.

One possible injection site is Juan de Fuca plate. Researchers at the Lamont-Doherty Earth Observatory found that this plate at the western coast of the United States has a possible storage capacity of 208 gigatons. This could cover the entire current U.S. carbon emissions for over 100 years.

This process is undergoing tests as part of the CarbFix project, resulting in 95% of the injected 250 tons of CO2 to solidify into calcite in 2 years, using 25 tonnes of water per ton of CO2.

Acid neutralization

Carbon dioxide forms carbonic acid when dissolved in water, so ocean acidification is a significant consequence of elevated carbon dioxide levels, and limits the rate at which it can be absorbed into the ocean (the solubility pump). A variety of different bases have been suggested that could neutralize the acid and thus increase CO
₂ absorption. For example, adding crushed limestone to oceans enhances the absorption of carbon dioxide. Another approach is to add sodium hydroxide to oceans which is produced by electrolysis of salt water or brine, while eliminating the waste hydrochloric acid by reaction with a volcanic silicate rock such as enstatite, effectively increasing the rate of natural weathering of these rocks to restore ocean pH.

Obstruction

Danger of leaks

Carbon dioxide may be stored deep underground. At depth, hydrostatic pressure acts to keep it in a liquid state. Reservoir design faults, rock fissures and tectonic processes may act to release the gas stored into the ocean or atmosphere.

Financial costs

The use of the technology would add an additional 1–5 cents of cost per kilowatt hour, according to estimate made by the Intergovernmental Panel on Climate Change. The financial costs of modern coal technology would nearly double if use of CCS technology were to be required by regulation. The cost of CCS technology differs with the different types of capture technologies being used and with the different sites that it is implemented in, but the costs tend to increase with CCS capture implementation. One study conducted predicted that with new technologies these costs could be lowered but would remain slightly higher than prices without CCS technologies.

Energy requirements

The energy requirements of sequestration processes may be significant. In one paper, sequestration consumed 25 percent of the plant's rated 600 megawatt output capacity.

After adding CO₂ capture and compression, the capacity of the coal-fired power plant is reduced to 457 MW.

Urban forestry

From Wikipedia, the free encyclopedia

Tree pruning in Durham, North Carolina

 

James Kinder, an ISA Certified Municipal Arborist examining a Japanese Hemlock at Hoyt Arboretum

 

Urban forestry is the care and management of single trees and tree populations in urban settings for the purpose of improving the urban environment. Urban forestry advocates the role of trees as a critical part of the urban infrastructure. Urban foresters plant and maintain trees, support appropriate tree and forest preservation, conduct research and promote the many benefits trees provide. Urban forestry is practiced by municipal and commercial arborists, municipal and utility foresters, environmental policymakers, city planners, consultants, educators, researchers and community activists.

Benefits

Professional Tree Climber (arborist: Zack Weiler) climbing a willow tree in Port Elgin, ON. Canada

 

Urban forests provide environmental, health, and economic benefits to cities. Urban forests mitigate the effects of urban heat island through evapotranspiration and the shading of streets and buildings. This improves human comfort, reduces the risk of heat stroke and decreases costs to cool buildings. Urban forests improve air quality by absorbing pollutants such as ozone, nitrogen dioxide, ammonia, and particulate matter as well as performing carbon sequestration. Urban forests are important to stormwater management. Trees absorb and store rainwater through the canopy, and slow down and filter runoff with their roots. Urban forests also encourage more active lifestyles by providing space for exercise and are associated with reduced stress and overall emotional well-being. Urban forests may also provide products such as timber or food, and deliver economic benefits such as increased property values and the attraction of tourism, businesses and investment.

The City of Denver Department of Parks and Recreation website hosts interactive online tools that allow residents to view the financial impact to their neighborhoods directly related to healthy tree planting. In the Washington-Virginia Vale neighborhood the city website cites 2,002 individual trees as having been planted and maintained by the City Forester. These trees are believed to bring in an annual ecosystem benefit of $159,521. This is mostly wrapped up in property benefits, which cite a contribution to this total of $143,331. The majorities of these trees are between 0 and 12 feet tall and are a mix of mostly Elm, Maple, Pine, and Locust species.

Mental health impacts

A 2018 study asked low income residents of Philadelphia "how often they felt nervous, hopeless, restless, depressed and worthless." As an experimental mental health intervention, trash was removed from vacant lots. Some of the vacant lots were "greened", with plantings of trees, grass, and small fences. Residents near the "greened" lots who had incomes below the poverty line reported a decrease in feelings of depression of 68%, while residents with incomes above the poverty line reported a decrease of 41%. Removing trash from vacant lots without installing landscaping did not have an observable mental health impact.

Practice

Urban forestry is a practical discipline, which includes tree planting, care, and protection, and the overall management of trees as a collective resource. The urban environment can present many arboricultural challenges such as limited root and canopy space, poor soil quality, deficiency or excess of water and light, heat, pollution, mechanical and chemical damage to trees, and mitigation of tree-related hazards. Among those hazards are mostly non-immediate risks like the probability that individual trees will not withstand strong winds (as during a thunderstorm) and damage parking cars or injure passing pedestrians. Although quite striking in an urban environment, large trees in particular present a continuing dilemma for the field of urban forestry due to the stresses that urban trees undergo from automobile exhaust, constraining hardscape and building foundations, and physical damage (Pickett et al. 2008). Urban forestry also challenges the arborists that tend the trees. The lack of space requires greater use of rigging skills and traffic and pedestrian control. The many constraints that the typical urban environment places on trees limits the average lifespan of a city tree to only 32 years – 13 years if planted in a downtown area – which is far short of the 150-year average life span of trees in rural settings (Herwitz 2001). 

Management challenges for urban forestry include maintaining a tree and planting site inventory, quantifying and maximizing the benefits of trees, minimizing costs, obtaining and maintaining public support and funding, and establishing laws and policies for trees on public and on private land. Urban forestry presents many social issues that require addressing to allow urban forestry to be seen by the many as an advantage rather than a curse on their environment. Social issues include under funding which leads to inadequate maintenance of urban trees. In the UK the National Urban Forestry Unit produced a series of case studies around best practice in urban forestry which is archived here.

By country

United States

History

Tree warden laws in the New England states are important examples of some of the earliest and most far-sighted state urban forestry and forest conservation legislation. In 1896, the Massachusetts legislature passed the first tree warden law, and the other five New England states soon followed suit: Connecticut, Rhode Island, and New Hampshire in 1901, Vermont in 1904, and Maine in 1919. (Kinney 1972, Favretti 1982, Campanella 2003).

As villages and towns grew in population and wealth, ornamentation of public, or common, spaces with shade trees also increased. However, the ornamentation of public areas did not evolve into a social movement until the late 18th century, when private individuals seriously promoted and sponsored public beautification with shade and ornamental trees (Favretti 1982, Lawrence 1995). Almost a century later, around 1850, institutions and organization were founded to promote ornamentation through private means (Egleston 1878, Favretti 1982). In the 1890s, New England's "Nail" laws enabled towns to take definitive steps to distinguish which shade trees were public. Chapter 196 of the 1890 Massachusetts Acts and Resolves stated that a public shade tree was to be designated by driving a nail or spike, with the letter M plainly impressed on its head, into the relevant trunk. Connecticut passed a similar law in 1893, except its certified nails and spikes bore the letter C. (Northrup 1887).

The rapid urbanization of American cities in the late 19th century was a concern to many as encouraging intellectual separation of humanity and nature (Rees 1997). By the end of the 19th century, social reformers were just beginning to understand the relationship between developing parks in urban areas and "[engendering] a better society" (Young 1995:536). At this time, parks and trees were not necessarily seen as a way to allow urban dwellers to experience nature, but more of a means of providing mechanisms of acculturation and control for newly arrived immigrants and their children (e.g., areas to encourage "structured play" and thus serve as a deterrent for youth crime) (Pincetl and Gearin 2005). Other prominent public intellectuals were interested in exploring the synergy between ecological and social systems, including American landscape architect Fredrick Law Olmsted, designer of 17 major U.S. urban parks and a visionary in seeing the value of including green space and trees as a fundamental part of metropolitan infrastructure (Young 2009). To Olmsted, unity between nature and urban dwellers was not only physical, but also spiritual: "Gradually and silently the charm comes over us; the beauty has entered our souls; we know not exactly when or how, but going away we remember it with a tender, subdued, filial-like joy" (Beveridge and Schuyler 1983 cited in Young 2009:320). The conscious inclusion of trees in urban designs for American cities such as Chicago, San Francisco, and Minneapolis was also inspired by Paris's urban forest and its broad, tree-lined boulevards as well as by the English romantic landscape movement (Zube 1973). The belief in green cover by early park proponents as a promoter of social cohesion has been corroborated by more recent research that links trees to the presence of stronger ties among neighbors, more adult supervision of children in outdoor areas, more use of the neighborhood common areas, and fewer property and violent crime (Kuo et al. 1998, Kuo and Sullivan 2001, Kuo 2003). 

Many municipalities throughout the United States employ community-level tree ordinances to empower planning officials to regulate the planting, maintenance, and preservation of trees. The development of tree ordinances emerged largely as a response to the Dutch Elm Disease that plagued cities from the 1930s to 1960s, and grew in response to urban development, loss of urban tree canopy, and rising public concern for the environment (Wolf 2003). The 1980s saw the beginning of the second generation of ordinances with higher standards and specific foci, as communities sought to create more environmentally pleasing harmony between new development and existing infrastructure. These new ordinances, legislated by local governments, may include specific provisions such as the diameter of tree and percentage of trees to be protected during construction activities (Xiao 1995). The implementation of these tree ordinances is greatly aided by a significant effort by community tree advocates to conduct public outreach and education aimed at increasing environmental concern for urban trees, such as through National Arbor Day celebrations and the USDA Urban and Community Forestry Program (Dwyer et al. 2000, Hunter and Rinner 2004, Norton and Hannon 1997, Wall et al. 2006). Much of the work on the ground is performed by non-profits funded by private donations and government grants.

Policy on urban forestry is less contentious and partisan than many other forestry issues, such as resource extraction in national forests. However, the uneven distribution of healthy urban forests across the landscape has become a growing concern in the past 20 years. This is because the urban forest has become an increasingly important component of bioregional ecological health with the expanding ecological footprint of urban areas. Based on American Forests' Urban Ecosystem Analyses conducted over the past six years in ten cities, an estimated 634,407,719 trees have been lost from metropolitan areas across the U.S. as the result of urban and suburban development (American Forests 2011). This is often due to the failure of municipalities to integrate trees and other elements of the green infrastructure into their day-to-day planning and decision-making processes (American Forests 2002). The inconsistent quality of urban forestry programs on the local level ultimately impacts the regional context in which contiguous urban forests reside, and is greatly exacerbated by suburban sprawl as well as other social and ecological effects (Webb et al. 2008). The recognition of this hierarchical linkage among healthy urban forests and the effectiveness of broader ecosystem protection goals (e.g., maintaining biodiversity and wildlife corridors), highlights the need for scientists and policymakers to gain a better understanding of the socio-spatial dynamics that are associated with tree canopy health at different scales (Wu 2008).

United Kingdom

In the UK urban forestry was pioneered around the turn of the 19th century by the Midland reafforesting association, whose focus was in the Black Country. England's Community Forests. programme was established in 1990 by the then Countryside Commission as a pilot project to demonstrate the potential contribution of environmental improvement to economic and social regeneration. Each Community Forest was established as a partnership between local authorities and local, regional and national partners including the Forestry Commission and Natural England. Collectively, this work has formed the largest environmental regeneration initiative in England. In the mid 1990s the National Urban Forestry Unit (NUFU) grew out of a Black Country Urban Forestry Unit and promoted urban forestry across the UK, notably including the establishment of the Black Country Urban Forest. As urban forestry become more mainstream in the 21st century, NUFU was wound up, and its advocacy role now carried on by organisations such as The Wildlife Trusts and the Woodland Trust.

Constraints

Resolving limitations will require coordinated efforts among cities, regions, and countries (Meza, 1992; Nilsson, 2000; Valencia, 2000).

• Loss of green space is continuous as cities expand; available growing space is limited in city centres. This problem is compounded by pressure to convert green space, parks, etc. into building sites (Glickman, 1999).
• Inadequate space is allowed for the root system.
• Poor soil is used when planting specimens.
• Incorrect and neglected staking leads to bark damage.
• Larger, more mature trees are often used to provide scale and a sense of establishment to a scheme. These trees grow more slowly and do not thrive in alien soils whilst smaller specimens can adapt more readily to existing conditions.
• Lack of information on the tolerances of urban tree cultivars to environmental constraints.
• Poor tree selection which leads to problems in the future
• Poor nursery stock and failure of post-care
• Limited genetic diversity
• Too few communities have working tree inventories and very few have urban forest management plans.
• Lack of public awareness about the benefits of healthy urban forests.
• Poor tree care practices by citizens and untrained arborists.

Eco-cities

From Wikipedia, the free encyclopedia

An eco-city or ecocity is "a human settlement modeled on the self-sustaining resilient structure and function of natural ecosystems", as defined by the Ecocity Builders (a non-profit organization started by Richard Register who first coined the term). Simply put, an eco-city is an ecologically healthy city. The World Bank defines eco-cities as “cities that enhance the well-being of citizens and society through integrated urban planning and management that harness the benefits of ecological systems and protect and nurture these assets for future generations”. Although there is no universally accepted definition of an 'eco-city', among available definitions, there is some consensus on the basic features of an eco-city. 

 

Eco-cities are commonly found to focus on new-build developments, especially in developing nations such as China, wherein foundations are being laid for new eco-cities catering to 500,000 or more inhabitants.

History

Origins

Initial ideas behind the eco-cities can be traced back to 1975 with the formation of a non-profit organization called Urban Ecology. Founded by a group of visionary architects and activists including Richard Register in Berkeley, California, the organization worked at the intersection of urban planning, ecology, and public participation to help formulate design concepts centered around building environmentally healthier cities. Some of their efforts included initiating movements to plant trees along the main streets, promoting the construction of solar greenhouses, developing environment-friendly policies by working with the Berkeley city planning division and encouraging public transportation. Building on these strategies, Richard Register later coined the term 'ecocity' in his 1987 book titled "Ecocity Berkeley: Building Cities for a Healthy Future" describing it as a city where human beings live in harmony with nature and therefore greatly reducing their ecological footprint. Urban Ecology began publishing articles focused on similar complex urban issues that elevated the movement further with the creation of their magazine, 'Urban Ecology' in 1987. For two decades, they also publish two newsletters, 'The Sustainable Activist' and 'The Urban Ecologist' to pursue their vision.

International Ecocity Conference Series (IECS)

Urban Ecology further advanced the movement when they hosted the first International Ecocity Conference in Berkeley, California in 1990. The conference focused on urban sustainability problems and encouraged over 800 participants from 13 countries to submit proposals on best practices to reform cities for a better urban ecological balance. 

Following this, in 1992, Richard Register founded the non-profit organization Ecocity Builders, to advance a set of goals outlined in the conference. Since its conception, the organization has been the convener of the International Ecocity Conference Series (IECS). The IECS has been the longest standing international conference series consisting of biennial Ecocity World Summits (EWS) and has been held in Adelaide, Australia (1992); Yoff, Senegal(1996); Curitiba, Brazil (2000); Shenzhen, China (2002); Bangalore, India (2006); San Francisco, United States (2008); Istanbul, Turkey (2009); Montreal, Canada (2011); Nantes, France (2013); Abu Dhabi, UAE (2015), and Melbourne, Australia (2017).

Other leading figures include architect Paul F Downton, who later founded the company Ecopolis Pvt. Ltd., as well as authors Timothy Beatley and Steffen Lehmann, who have written extensively on the subject.

Current Trends

Eco-cities Criteria

An ideal eco-city has frequently been described as one that fulfills the following requirements:

An example of a green roof project

• Operates on a self-contained economy that obtains resources locally
• Is entirely carbon-neutral by promoting techniques like the use and production of renewable energy
• Is established over a well-planned city layout that promotes walkability, biking and the use of public transportation systems
• Promotes conservation of resource by maximizing water efficiency and energy efficiency, while managing an ecologically beneficial waste management system that promotes recycling and reuse to create a zero-waste system
• Restores environmentally damaged urban areas
• Ensures decent and affordable housing for all socio-economic and ethnic groups and improves jobs opportunities for disadvantaged groups, such as women, minorities, and the disabled
• Supports local agriculture and produce
• Supports future progress and expansion over time.

Besides these, each individual eco-city has an additional set of requirements to ensure ecological and economic benefits that may range from large-scale targets like zero-waste and zero-carbon emissions, as seen in the Sino-Singapore Tianjin Eco-city project and the Abu Dhabi Masdar City project, to smaller-scale interventions like urban revitalization and establishment of green roofs as seen in the case of Augustenborg, Malmö, Sweden.

Ecocity Framework and Standards Initiative (EFSI)

With a growing popularity of the concept, in the last few decades, there has been an exponential growth in the number of eco-cities established around the globe. To assess the performance of these eco-cities and provide future guidance, the Ecocity Framework and Standards Initiative (EFSI), established by Richard Register's Ecocity Builders and the British Columbia Institute of Technology (BCIT) School of Construction and the Environment, provides a practical methodology for this to ensure progress towards the intended goals of eco-cities. The four pillars in this framework include:

• Urban Design (containing 4 criteria for access by proximity)
• Bio-geo Physical Features (containing 6 criteria for the responsible management of resources and materials as well as the generation and use of clean, renewable energy)
• Socio-cultural Features (containing 5 criteria for promoting cultural activities and community participation)
• Ecological Imperatives (containing 3 criteria to sustaining and restoring biodiversity)

Using these, the International Eco-Cities Initiative recently identified and rated as many as 178 significant eco-city initiatives at different stages of planning and implementation around the world. To be included in this census, initiatives needed to be at least district-wide in their scale, covering a variety of sectors, and have official policy status. Although such schemes display great variety in their ambitions, scale, and conceptual underpinnings, since the late 2000s there has been an international proliferation of frameworks of urban sustainability indicators and processes designed to be implemented across different contexts. This may suggest that a process of eco-city 'standardization' is underway.

Practical Limits

Richard Register once stated that "An ecocity is an ecologically healthy city. No such city exists". Despite the conceptual ecological benefits of eco-cities, actual implementation can be difficult to attain. The conversion of existing cities to eco-cities is uncommon because the infrastructure, both in terms of the physical city layout and local bureaucracy, are often major insurmountable obstacles to large-scale sustainable development. The high cost of the technological integration necessary for eco-city development is a major challenge, as many cities either can’t afford, or are not willing to take on, the extra costs. Such issues, along with the added challenges and limits to retrofitting existing cities contribute to the establishment of newly constructed eco-cities. Along with this, the costs and infrastructure development needed to manage these large scale, two-pronged projects extend beyond the capabilities of most cities. In addition, many cities around the world are currently struggling to maintain the status quo, with budgetary issues, low growth rates, and transportation inefficiencies, that encourage reactive, coping policies. While there are many examples worldwide, the development of eco-cities is still limited due to the vast challenges and high costs associated with sustainability.

Related Terminologies

Eco-cities have been developed as a response to present-day unsustainable systems that exist in our cities. Simultaneously, there have been other concepts like Smart Cities, Sustainable Cities, and Biophilic Cities that also strive towards achieving sustainability in cities through different approaches. Owing to ambiguity in their definitions and closely related criteria defined to achieve their goals, these concepts, despite their varying approaches, are often used interchangeably.

Criticism of the Eco-city Concept

The Three Pitfalls

Looking at the patterns of progress in the last few decades of city construction towards sustainability, Valaria Saiu (University of Cagliari) poses one major criticism through the existence of a theory-practice gap caused by economic and ethical conflicts and risks that generate socio-spatial utopias. She identifies three pitfalls in the concept of sustainable cities (and therefore, eco-cities):

1. The Idea of the City as a Business: "Most eco-city projects are dependent on technologies available on the global market and the city is considered as a big economic affair". Often developed as techno-centric concepts, these projects seek investment opportunities by public-private partnerships leading to a top-down approach. This structure lacks democratic approaches in the decision-making process which further contributes to running high risks of failure, especially in social terms.
2. The Oversimplification of Urban Complexity: Due to the nature of current trends in measuring sustainability, there has been a strong focus in the quantifiable aspects of sustainability like energy-efficiency or waste-efficiency. This creates a tendency of oversimplification by neglecting the social and political aspects of the city that are unmeasurable qualitative aspects, yet significant to the fundamental concept of eco-cities.
3. The Quest for the Ideal Community: This section of the criticism focuses on the practical limits to merging economic goals with social goals in the urban development process. “Under the banner of green technology, inhabitants are forced to pay higher costs for their use of facilities in eco-cities.”

Eco-Cities as Isolated Entities

Another larger conceptual criticism faced by eco-cities stems from the ambiguity in the definition of sustainability as a term. This has been further elaborated by Mike Hodson and Simon Marvin in their article titled 'Urbanism in the Anthropocene: Ecological Urbanism or Premium Ecological Enclaves' where they noted "We have tended to refer to sustainability in a generic sense, and our discussions of sustainability could be employed to anything that has sustainable as an adjective". As a result of this, a widespread trend has been observed in the growing number of eco-cities developed over the past two decades that claim to combat our current global climate-change challenges. Many of these cities are found to be established in isolation from other existing urban centers due to the nature of their ownership. Owing to this isolation, internalization of resource-flows contribute towards a shallow sense of ecological sustainability in such cities.

Urban Ecological Security (UES) and the Social, Economic and Environmental Impacts of Eco-Cities

Eco-cities have also been criticized to have biases towards the economic and environmental pillars of sustainability while neglecting the social pillar. The practical translations of the concept have faced criticism as eco-cities have been driven by the demand for bounded ecological security. By offering "premium ecological enclaves" factoring ecological security as an outcome of private investments driving the construction of eco-cities, the existing examples of eco-cities are criticized for not being truly sustainable solutions. On the contrary, by placing this concept in the meta-narrative of sustainable cities, these have also been further criticized for celebrating this fragmentation of society through the development of gated communities and premium ecological enclaves isolated from the real global scale of issues in today's ecological crisis. For instance, the eco-cities of Masdar and Hong Kong pose homogeneous visions, but have been criticized to be the source of fragmentation of urban society. 

The term "Frankenstein Urbanism" was used by Federico Cugurullo to metaphorically symbolize this criticism of the concept that increases social stratification in exchange for ecological security, creating isolated entities that could work perfectly within themselves, but fall apart when brought in a larger view.