Biogeophysical effects of land use on climate: Model simulations of radiative forcing and large-scale temperature change
Introduction
Forty-four to forty-nine million hectares of the global land surface are currently classified as either crops, pasture or grazing, meaning that at least 34% of the global land surface is subject to direct alteration by human activities. As well as affecting climate through absorption or emission of greenhouse gases (biogeochemical effects), land cover change can influence climate by modifying the physical properties of the land surface (biogeophysical effects). The nature of the vegetation affects the surface fluxes of radiation, heat, moisture and momentum, so modifying the vegetation cover can change the lower boundary conditions of the atmosphere and hence impact the climate (Pielke et al., 1998). Conversion of forest to cropland or pasture reduces the aerodynamic roughness of the landscape and decreases both the capture of precipitation on the canopy and the root extraction of soil moisture; these changes tend to decrease evaporation and hence reduce the fluxes of moisture and latent heat from the surface to the atmosphere, which acts to increase the temperature near the surface (Lean and Rowntree, 1993). Also, a forested landscape generally has a lower surface albedo than open land, particularly in conditions of lying snow when shortwave radiation is trapped by multiple reflections within the forest canopy (Betts and Ball, 1997). Deforestation can therefore lead to increased shortwave reflection, which provides a cooling influence (Thomas and Rowntree, 1992, Bonan et al., 1992, Douville and Royer, 1997).
The relative importance of these processes depends on local conditions such as the underlying surface albedo and soil moisture availability, and can vary with season and location (Betts, 1999). Surface albedo change may provide the dominant influence of mid- and high-latitude land cover change on climate (Bonan et al., 1992, Betts, 1999, Bounoua et al., 2002), with deforestation causing cooling, whereas in the moist tropics the main effect may be via evapotranspiration with deforestation causing warming. In the 1970s, before concern about anthropogenic greenhouse-forced climate change became widespread, Sagan et al. (1979) suggested that humans had exerted an overall cooling influence by increasing global albedo. Brovkin et al. (1999) used an Earth System Model of Intermediate Complexity to simulate the climatic effects of anthropogenic land cover change from the potential natural state to the present-day, and found that overall biogeophysical impact of global land cover changes was a cooling as suggested by Sagan et al. (1979). Betts (2001) and Govindasamy et al. (2001) used General Circulation Models (GCMs) of climate for similar studies and found similar results. Here we discuss the results of Betts (2001) and compare them with other studies.
Perturbations to the Earth's radiation budget, such as changes in albedo, can be compared directly with the effects of greenhouse gases and aerosols through the concept of radiative forcing. This can be thought of as a perturbation to the global radiation budget prior to any feedbacks resulting from the response of other aspects of the climate system, and is conventionally defined as the change in the net flux at the tropopause. Since the radiation budget is the fundamental driver of the climate system, the concept of radiative forcing can provide a useful indicator of the relative importance of the different mechanisms of global temperature change. The radiative forcing concept has its limitations; for example, climate sensitivity to a given level of forcing can vary by 50% or more depending on the characteristics of the forcing and the altitude at which the radiative flux changes act (Hansen et al., 1997a, Christiansen, 1999). Furthermore, radiative forcing cannot be used to quantify all mechanisms of climatic perturbation, such as those which act directly via surface moisture fluxes. Nevertheless, when used with care, radiative forcing can provide a means of comparing the influence of different perturbation mechanisms on climate. Land-use-induced surface albedo change perturbs the radiation budget by modifying the absorption of shortwave radiation; this shortwave radiative forcing can then be compared with the radiative forcings by greenhouse gases, aerosols and solar output changes to assess the importance of surface albedo change in relation to these other climatic influences.
Radiative forcings due to human activities are frequently quantified with relation to the start of the industrial era, typically considered as 1750 (e.g. Ramaswamy et al., 2001), since this was the time at which anthropogenic emissions of greenhouse gases became significant. However, since agriculture was already widespread in parts of Europe and Asia by this time (Ramankutty and Foley, 1999, Klein Goldewijk, 2001), the present-day forcing relative to 1750 is likely to be different to the forcing relative to naturally vegetated state. A comparison of land use forcing with other radiative forcings therefore requires an estimate of the land use forcing relative to the vegetated state at the start of industrial era (circa 1750). Here we provide such an estimate. We use reconstructions of past land cover to perform a spatially explicit simulations of radiative forcing at 1750, 1850, 1900, 1950 and 1990 relative to natural (hereafter “NAT”). These show the magnitude and pattern of radiative forcing prior to fossil fuel burning, how these have changed over the last 250 years, and allow diagnosis of radiative forcing at 1990 relative to 1750.
The pre-industrial era (pre-1750) is widely regarded as being free from anthropogenic influence on climate. However, since large parts of Europe and south-east Asia had already been deforested before the industrial revolution, the resulting change in surface albedo could already have been exerting a radiative forcing of climate by the time fossil fuel burning began. Therefore, it may not be realistic to describe the pre-industrial era as “pre-anthropogenic”. Here, using our simulations of radiative forcing at 1750 relative to NAT, we estimate the extent of this pre-industrial anthropogenic influence.
Finally, we discuss the implications of land-use induced albedo changes for the use of forestry activities for reducing greenhouse-forced climate change. The United Nations Convention on Climate Change (UNFCCC) aims to limit anthropogenic climate change, and the Kyoto Protocol to the UNFCCC provides a framework for measuring and monitoring this in terms of the net carbon emissions of the parties to the convention. The Kyoto Protocol allows parties to use carbon sequestration by afforestation and reforestation to offset emissions of fossil fuels when calculating net carbon emissions (UNFCCC, 1997), implying that net carbon emissions are representative of the net effect on climate. However, increased forest cover also decreases surface albedo, especially in snowy regions, exerting warming which could partly or wholly offset the cooling due to CO2 removal through carbon sequestration (Betts, 2000). Here we compare these effects in terms of radiative forcing and compare with other studies of this issue.
This paper presents or reviews a number of climate modelling studies investigating the following questions concerning the biogeophysical impacts of land cover change on climate.
- (1)
What is the overall biogeophysical impact of current human-induced land surface changes on global climate?
- (2)
Is albedo change the main mechanism for this impact?
- (3)
How does albedo change compare with other human influences on climate over the past 250 years?
- (4)
Was human-induced albedo change prior to the industrial revolution large enough to provide a significant impact on climate before the period normally considered as being subject to human interference?
- (5)
Do the impacts of forests on albedo affect their usefulness in reducing greenhouse-forced climate change?
Questions 1 and 2 are addressed with full climate simulations. Questions 3–5 are addressed using simulations and estimates of radiative forcing. Changes in greenhouse gas concentration exert a forcing primarily in the infrared part of the spectrum (longwave), whereas surface albedo change exerts a forcing at wavelengths in and around the visible region (shortwave). A positive radiative forcing implies a warming influence, and a negative forcing implies cooling.
The above questions are addressed with a series of three related studies:
- (a)
Impact of current land use on climate. Two climate model simulations were performed with land surface parameters representing current and natural vegetation, and a third was performed with only the albedo parameters representing natural vegetation while the remaining parameters were set to the current vegetation state (Section 3).
- (b)
Simulation of the radiative forcing due to land use change up to the present-day (Section 4). Within a simulation of present-day climate, the radiation sub-model was used to simulate radiative forcing due to land use change relative to natural vegetation for 1750, 1850, 1900, 1950 and 1990.
- (c)
Simulation of the radiative forcing due to reforestation and comparison with the estimated radiative forcing due to carbon sequestration (Section 5). Again within a simulation of present-day climate, the radiation sub-model was used to simulate the local radiative forcing due to afforestation at any point where forest can currently be supported.
We discuss the key implications and common messages from these three studies, and present recommendations for the consideration of the biogeophysical effects of land use change within policy-relevant climate change science.
Section snippets
The Hadley centre climate model
The climate and radiative forcing simulations were performed with the HadAM3 Atmospheric General Circulation Model (Pope et al., 1999). This simulates global atmospheric and land surface processes at a horizontal resolution of 2.5° × 3.75°, using the Edwards and Slingo (1996) radiative transfer scheme and the Met Office Surface Exchange Scheme (MOSES) land surface model (Cox et al., 1999). Together these parameterize surface albedo using vegetation-dependent parameters and snow depth (Hansen et
Climatic effects of present-day land use relative to the potential natural vegetation state
To simulate the biogeophysical effects of current land use on climate, three simulations were performed with HadAM3. The first used the WHS dataset for present-day vegetation, and the second used the NAT reconstruction of potential natural vegetation. In each of these two simulations, the land use scenario was used to derive datasets of all the land surface parameters required by the MOSES land surface scheme, and the atmosphere responded fully to all of these. In the third simulation, only the
Radiative forcing due to historical surface albedo changes
The concept of radiative forcing can be used to compare the climatic influences of different drivers without the additional uncertainties associated with the climate sensitivity of the model. Since albedo change appears to be the dominant biogeophyical effect of past land use change on temperature at the global scale, diagnosis of the radiative forcing due to surface albedo change is useful for comparing the mean climatic impacts of historical land cover change with other large-scale
Implications of biophysical effects for carbon sink plantations
Afforestation and reforestation feature prominently among proposals for mitigating climate warming (UNFCCC, 1997), due to their potential to sequester carbon from the atmosphere. However, since forests also influence climate though biogeophysical mechanisms, the overall influence of carbon sink forest plantations on climate could be somewhat different to that expected on the basis of carbon cycle impacts alone. In particular, since high-latitude forests exert a warming influence on climate
Discussion and conclusions
The first study discussed here suggests that historical deforestation has exerted an overall cooling influence, because most deforestation has taken place in temperate regions where the dominant influence is through an increase in surface albedo particularly in winter and spring. In these seasons, the 1.5 m temperature simulated in the Eurasian and North American agricultural regions is up to 2 K lower with actual rather than potential vegetation, and the annual mean cooling is approximately 0.5–1
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