Creating urban greenspace can help mitigate local climate change by helping to cool the climate through evapotranspiration from plants, shading effects, and rainfall interception and percolation. The shade and cooling effects have an additional beneficial knock-on effect of reducing demand for air conditioning in hot temperatures – with both local benefits in reduced heat transfer from buildings to the local environment, and global benefits in reducing energy use. Other secondary benefits of urban greenspace may be to encourage more local recreation rather than travelling out to the countryside, thus reducing energy consumption.
The changing climate affects all aspects of the environment – including urban greenspaces – both directly and indirectly. Direct effects may be caused by changes to the environment for trees, other vegetation and the rest of the ecosystem. The climatic conditions for the growth of trees and other vegetation in urban areas across the UK have already changed noticeably in the past 50 years. Scenarios for climate change driven by human activities (mainly the burning of fossil fuels and deforestation) suggest there will be more substantial alterations in the near future (IPCC, 2007). These are mainly changes in temperature and rainfall, but also include effects on sunshine, cloudiness, humidity and wind speed, and on concentrations of atmospheric gases including CO2.
Indirect effects include those that affect human behaviour and policy. A warmer climate may mean we need less heating but more air conditioning, changing urban atmospheric pollution. Higher temperatures may create more demand for greenspace for recreation. Policies designed to reduce carbon emissions may result in changes to urban transport; or in modifications to planning targets, such as altered housing density; or in better protected floodplain areas.
The urban environment is warmer than the countryside (2°C or more), particularly at night, depending on the local setting and the size of the urban area. There are also pronounced differences in microclimate due to variations in aspect and soil substrate. The design of urban greenspace already takes these factors into account.
Changes to the macroclimate will affect the microclimate, altering the growth of trees and other vegetation, the natural processes within soils, and the growth of animals dependent on the vegetation and soils. Some species will become less well adapted to the new conditions, and others will be better adapted. In managed environments, such as many urban greenspaces, this will have a major effect on the choice of plant species to grow, on establishment practices, and on the management of vegetation, for example control of weed growth.
Pests and diseases may be altered in many ways – the types and species of outbreaks, the frequency and severity are all likely to be modified, and this links to considerations about biosecurity.
Where greenspace is being created, the processes of soil formation may be altered, and climate change may affect the choice of substrate for the creation of new soil and for its subsequent management.
There are also likely to be strong pressures for the creation of more greenspace to improve the urban climate through shade and evaporative cooling, run-off reduction, and pollutant filtering.
Urban trees can also play a small part in energy generation, for example if prunings and felled material are used in biomass combustion, for example in local combined heat and power plants. But given the area and volume involved, this will not be a large contribution.
Trees have an important role to play in helping society to adapt to climate change, particularly in the urban environment. In order to help adaptation to climate change the Read report(PDF-2960K) suggests increasing the tree cover in the UK by 4% to 16% by 2050. On an annual basis, this would lead to emissions abatement of equivalent to 10% of total predicted greenhouse gas (GHG) emissions at that time. As a result of the Read report Defra’s climate change plan for 2010 (PDF-8220K) illustrates Defra’s commitment to creating more woodland in England, and to maximising their contribution in adaptation to climate change.
Guidance about choosing urban tree species to take account of climate change is becoming available – for example, the Greater London Authority’s Right Trees for a Changing Climate. This guidance can be tailored for other locations, based on local experience and the prevailing climatic conditions.
Urban greenspace can also provide an information and educational resource about climate change. People’s perceptions of the environment and how it is changing are usually informed by the changing seasonal wildlife patterns they experience. One example is the nationwide network of voluntary phenological observations (see Nature’s Calendar), such as flowering and leafing dates – the observations recorded are predominantly urban, reflecting the location of most people.
Right Trees for London’s Changing Climate
Trees are a very important part of the urban environment. In Greater London there are around 7 million trees; a quarter of these are in woodlands, which occupy 8% of London’s land area. An estimated 20% of London’s land area is under the canopy of individual trees (GLA 2005).
The predictions for climate change for London include substantial warming and more seasonal rainfall distribution, including wetter, milder winters and drier, hotter summers. These changing conditions will markedly change the suitability of tree species – existing species may become less successful, and the trees being planted now need to be appropriate for future conditions.
One of the initiatives under the umbrella of the Mayor's London Tree and Woodland Framework is the Right Trees for a Changing Climate, which involves a number of partners including Forest Research. Research has been carried out on better prediction of species survival and growth under changing climate, and practical guidance has been developed on how best to adapt to forthcoming climate changes.
Forest Research prepared the underlying database of tree species characteristics that will be suitable for the climatic conditions predicted for London and other urban areas over the rest of this century. The web-based database helps planners, landscape designers, developers, ecologists and other professionals decide what trees are suitable to plant in London and other urban areas in the face of a changing climate.
- the impacts of climate change;
- how trees and woodlands can help in mitigating climate change;
- how we can adapt trees, woodland and their management for future climates.
Our work and programmes range from tree species suitability assessments and assessing pest and disease risk to the role of trees in flood control.
Forest Research has considerable expertise in monitoring and modelling tree establishment and growth, and in monitoring many aspects of the environment, including soil physical and chemical conditions, air quality and many aspects of the microclimate (see Woodlands and the environment). We are also conducting research into people’s interactions with, and understanding of, trees and woodland (see People, trees and woodlands).
Broadmeadow, M. (2004a). Woodland and our Changing Environment (PDF-2510K). Sustainable Forestry In Brief. Forestry Commission, Scotland.
Broadmeadow, M. (2004b). The Potential Effects of Climate Change for Trees and Woodland in the South West (PDF-189K). Forest Research, Farnham.
Broadmeadow, M. and Matthews, R. (2003). Forests, Carbon and Climate Change: The UK Contribution (PDF-1520K). Information Note 48. Forestry Commission, Scotland.
Broadmeadow, M., Ray, D., Sing, L. and Poulsom, L. (2004). Climate Change and British Woodland: What does the Future Hold? (PDF-910K). Forest Research Annual Report and Accounts 2002–03. Forestry Commission, Scotland.
Defra (2007). A Strategy for England’s Trees, Woods and Forests (PDF-2640K). Department for Environment, Food and Rural Affairs, London.
Defra (2010). Defra’s climate change plan 2010(PDF-8220K). Department for Environment Food and Rural Affairs, London.
Freer-Smith, P.H., Broadmeadow, M.S.J. and Lynch, J.M. (eds) (2007). Forestry and Climate Change. CABI Publishing, Wallingford.
GLA (2005). Connecting Londoners with Trees and Woodlands: A Tree and Woodland Framework for London(PDF-1550K). Greater London Authority, London.
Hubert, J. and Cottrell, J. (2007). The Role of Forest Genetic Resources in Helping British Forests Respond to Climate Change (PDF-959K). Information Note 86. Forestry Commission, Edinburgh.
IPCC (2007). Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, Pachauri, R.K and Reisinger, A. (eds.)]. IPCC, Geneva.
Ray, D. (2008). Impacts of Climate Change on Forestry in Scotland – A Synopsis of Spatial Modelling Research (PDF-920K). Research Note 101. Forestry Commission, Scotland.
Read, D.J., Freer-Smith, P.H., Morison, J.I.L., Hanley, N., West, C.C. and Snowdon, P. (eds). (2009). Combating climate change – a role for UK forests. An assessment of the potential of the UK’s trees and woodlands to mitigate and adapt to climate change (PDF-2960K). The synthesis report. The Stationery Office, Edinburgh.