Land Cover Change and Climate

Stephanie J. Houser

April 2006

1. Introduction

Climate change is a widely studied phenomenon. Today, the impacts that human lifehave on our climate are become increasingly important as we look towards and preparefor the future. Global warming is a highly debated topic as emissions of CO2 areincreasing at an exponential rate, and are causing more and more warming each year.The impact of CO2 on climate is pretty well understood and is included in most reportson future climate scenarios. One important factor that is somewhat overlooked is therole that land-cover change has on climate variability. As human life continues tocovert land from one extreme to another, regional and global climates will continue tochange. Although the IPCC has yet to include these effects in their future climatereports, increasingly more studies are pointing to how crucially important it is toinclude these land-cover changes and their effects in order to better predict how ourclimate will change in the following years.

2. The Importance of Land Cover Change

During the last millennium, humans have transformed natural ecosystems, such asforests and grasslands, into croplands, pastures, and bare soil (Brovkin, 1999). Theseland cover changes will continue into the future and will certainly have an impact onthe climate. The Florida peninsula has converted from mostly forested land in pre-1900 to a crop covered land by 1993 (Figure 1). This type of land-cover change isoccurring in other regions around the world as well. Whether land-cover changes are adirect result of human activity, or occur naturally, their impacts on future climate areimportant. The importance of these impacts is becoming more clear as aditionalresearch is being conducted on the topic. However, the exact magnitude of importanceof land cover change on future climates is still being debated. Roger Pielke Sr. arguesthat “land use effects may be at least as important in altering the weather as changes inclimate patterns associated with greenhouse gases” (Pielke, 2005). The conversion from one land-cover type to another is more often than not apermanent change. Thus any impact on the climate due to this change would not beshort lived; rather it would be a long term effect having lasting effects on the climate(Pielke, 2002). It seems the magnitude of importance of land-cover change on futureclimate can only be determined by knowing and understanding how certain landconversions directly impact climate on both regional and global scales. However, it isoften times hard to pick out global effects as regional effects of opposite signfrequently cancel each other out, thus impacts need to be assessed more closely on aregional scale (Pielke, 2002).

3. The Impacts of Land Cover Change on Climate

Different forms of land-cover change have different effects on climate. These impactscan be divided into two major categories: biogeochemical and biogeophysical(Feddema, 2005). Biogeochemical processes change the chemical composition of theatmosphere. The amount of CO2 in the atmosphere can be directly affected by theregional climate. Where trees often help to sequester CO2 and decrease the amount ofit in the atmosphere, a deforestation conversion will act to increase the amount of CO2locally. Bigeophysical processes affect the physical parameters that determine theamount of solar energy that is absorbed by the Earth’s surface, as well as how thisenergy is distributed at the Earth’s surface. The most important of these parametersand the one that is looked at most often with respect to climate is albedo. The albedo ofa forested region is less than that of an open grass field or desert. Changing the albedoby converting the land-cover, changes the amount of energy absorbed and reflected outof the region, affecting the local climate. Two main land-cover changes are deforestation and reforestation. Deforestation isthe conversion of forest land to non-forestland, which is often agriculture. This processcauses a regional warming. Evapotranspiration due to transpiration off of tree leaves,and evaporation from the soil, causes a cooling in forested regions. Removing thiscooling source will act increase the temperature locally, thus causing warming.Deforestation also leads to a modification of surface fluxes of both heat and moisture.With a decrease in moisture flux from the surface, cloud cover will likely decrease,which will allow for more incident radiation to reach the surface. This process willlead to a net warming at the surface (Feddema, 2005). Forestation and reforestation can have opposite impacts on climate depending on thelatitude where either process occurs. Planting forests can lead to cooling by increasingthe amount of evapotranspiration locally. This increase in moisture flux from thesurface can lead to an increase in local cloud cover which will act to decrease theamount of incident radiation. This cooling effect dominates at low latitudes asevapotranspiration cools the surface more efficiently due to the exponential increase ofsaturation water vapor pressure with increasing temperature (Gibbard, 2005).However, at higher latitudes, which are prone to higher snowfall amounts, forestationor reforestation can have a warming effect. The snow albedo will be lowered whentrees are present as the snow falls through the trees to the surface below, and thecanopy above reduces the reflectance of the snow surface as compared to a snowcovered bare ground. This increases the amount of incident solar radiation, thuscreating a warming effect. Gibbard et. al. performed a study to illustrate this differencein impact based on latitude which will be discussed in more detail in the followingsection.

4. Climate Model Studies

Recent studies have been done to try and simulate the effects of land use and land-cover change on climate, as well as the magnitude of impact these changes will have.Two recent studies were done by Feddema et. al., and Gibbard et. al. Each study looksat the impact of land-cover change on climate using different climate models anddifferent scenarios.

a. Feddema Et. Al., 2005

The goal of this study was to simulate the effects of land-cover change on climateusing projected land-cover changes under the B1 and A2 IPCC SRES emissionscenarios. Feddema et. al. used the coupled Department of Energy Parallel ClimateModel (DOE-PCM) to simulate land-cover forcings and atmospheric forcings for boththe B1 and A2 scenarios. In order to simulate land-cover change, land-coverprojections were made using the Integrated Model to Assess the Global Environment(IMAGE 2.2) IPCC SRES emission scenarios, along with natural vegetation from theDOE-PCM data. The study projected land-cover changes from the present land-coverfor the years 2050 and 2100 for both the B1 and A2 scenarios (Figure 2). Then usingthe projected land-cover changes, the model ran with present day land-cover for 2000-2033, then the 2050 projected land-cover change was used for the years 2033-2066,and for the final years, 2066-2100, the 2100 projected land-cover change was used torun the model. In order to determine the changes due to land forcings alone, a control run with onlyatmospheric forcings with the present day land-cover held constant was also done.Then these results were subtracted from the model run results which included the landforcings due to land-cover change as well as the atmospheric forcings. The resultsshow that the midlatitude and tropical regions experience the most significant climateimpacts due to land-cover conversions. The results mainly focus on temperature changes due to regional land-cover changesfor both scenarios during the winter and summer seasons (June, July, August, andDecember, January, February). Looking at JJA temperature changes, the mostsignificant warming is seen over the Amazon for the A2 2100 scenario, while somesignificant areas of cooling include the southwestern US and India (Figure 3). Thewarming that occurs over the Amazon region is explained by the deforestation that wasprojected for the area by 2100. A conversion to agriculture in this tropical regionwould lead to less evapotranspiration, as well as a decrease in cloud cover due to thedecrease in moisture flux from the surface. Together, these modifications would leadto a net warming of the surface temperatures over the Amazon as more incidentradiation is available to warm the surface. Instead of increasing daily maximumtemperatures, this land change effect acts to increase the daily minimum temperaturesof the Amazon, decreasing the diurnal temperature range (Figure 4). However, similarland-cover changes were projected for Indonesia, yet minor temperature responses areprojected by the model. Feddema et. al. explains that the reason Indonesia does notexperience such a warming is due to the increase in local rainfall over the region dueto the Asian Monsoon. The monsoon provides an increased amount of water whichleads to increased evaporation rates, thus offseting the increase in sensible heat fluxfrom the surface as well as the increase of temperatures. Feddema et. al. suggests that the Amazon deforestation and cooler temperatures overthe surrounding oceans leads to a weakening of the Hadley circulation. This thenallows for a more northward migration of the Intertropical Convergence Zone (ITCZ)which leads to more intense southwest monsoon precipitation over the region. Thisincrease in moisture availability leads to an increase in latent heat flux, which acts tocool local daytime temperatures, thus decreasing the diurnal temperature range over theregion (Figure 4). This helps to explain the cooling over the region in the initialtemperature change results.

It seems that some of the model results from this study are not easily explained, nordo they seem to make sense. Thus Feddema points out that further study and researchis needed to better understand the results of this climate model study.

b. Gibbard Et. Al., 2005

This study used the 3rd version of the Community Land Model (CLM3) in its offlinemode. The standard vegetation types of the model were then replaced with a singlevegetation type in each grid cell, without regard to whether or not it was possible forthat vegetation type to grow in every region. Normal lake and glacier percentages werekept in their corresponding grid cells. The model was run in offline mode for 20 years,along with a controlled run using the standard vegetation type of the CLM3 model forcomparison. The goal of this study was not to produce possible land change scenariosand their resulting climate changes, but instead to try and determine the magnitude oftemperature changes that would occur due to certain global land conversions to onetype of vegetation. Taking the results of the 2-meter air temperature for each single vegetation type, andsubtracting from it the results for the bare ground run, it is obvious that the localizedsurface temperature anomalies due to land-cover change are dependent on latitude.The results show that with any type of vegetation, cooling occurs at the lower latitudes,and warming occurs at the higher latitudes (Figure 5). The warming at higher latitudesis attributed to the lowering of surface albedo with respect to bare ground. The coolingin the lower latitudes is due to the evapotranspiration process dominating due to thewarmer temperatures of the region. Based on the results of the offline model runs,another study was conducted using a coupled model considering only tree-typevegetation. The coupled model simulations used the CAM3 atmosphere model coupled with theCLM land model, along with a slab ocean and thermodynamic sea ice model. The typeof tree coverage was determined by the average tree type for each latitude band, andthe most common type was used for all land masses at that latitude. The coupled treemodel was run for 50 years, as was the control run with bare soil globally. The results of the coupled model experiment show similar results as the offlinemodel runs. Strong warming is seen in the upper latitudes which are mostlycategorized by boreal forest (Figure 6a). This warming can be attributed to thedecrease in albedo of snow prone areas since the albedo of snow covered forest is lowerthan the albedo of snow covered bare soil. Cooling seen in the tropical regions canonce again be explained by the evapotranspiration process dominating over the albedoeffect in regions of higher temperatures. Another notable result of the coupled run withglobal tree cover is the downwind heating and cooling off of the land masses (Figure6a). Westerly winds at the midlatitudes cause warming off the east side of thecontinents, where the tropical easterlies cause cooling off the west coast of SouthAmerica. In order to determine if the intense warming due to trees was exclusively due to theboreal forests of the higher latitudes, another run was executed replacing only themiddle latitudes, 30-50°N, with average tree cover while the rest of the land was keptat the current surface coverage. The results did show a significant warming at themidlatitudes, thus the overall warming poleward of the tropic regions can be attributed

to all tree type cover and not simply the boreal forests of the upper latitudes (Figure6b). This warming is clearly due to the albedo difference between forests and bare soil.As stated before, snow-covered vegetation has a lower albedo than snow-covered baresoil. This resultant warming leads to a decrease of snowfall in the region, which thenleads to a decrease in surface albedo, allowing for more incident radiation to beabsorbed at the surface, increasing the surface temperatures. This snow-albedofeedback effect seems to be responsible for some of the albedo changes, especially inthe northern latitudes where snowfall is decreased due to increased temperatures fromtree cover (Figure 6c). However, Gibbard et. al. notes that with a warmer climate inthe future, this albedo effect will be less obvious. Gibbard et. al. continues on to discuss the effects of cooling due to carbonsequestration by reforestation. This model study suggests that the cooling due tocarbon sequestration by trees is offset by the warming due to the same trees. Also it isnoted that the perturbations of CO2 to the atmosphere would hit equilibrium with theocean and rock cycles, whereas the albedo change would likely be permanent. Thuscooling due to carbon storage by trees would dominate on the decadal scale, whilewarming associated with the long term albedo change would dominate on more of acentury time scale (Gibbard, 2005).

5. Summary

Changes in land-use and land-cover are important drivers of climate. Land changes areoften permanent and therefore the climate changes induced by them are likely to belong term changes. Although it is sometimes hard to see the global effects of landchange on climate due to regional effects of opposing sign being canceled out, theimpacts on regional climates are of importance and can effect large scale circulations. Climate studies have shown that forestation in the mid to upper latitudes will lead towarming as the snow-albedo, as well as the surface-albedo, decrease. This allows formore incident radiation to be absorbed at the surface leading to higher surface heatfluxes and higher temperatures. On the other hand, forestation can also lead to cooling.This effect is seen in the tropics where evapotranspiration processes dominate withhigher temperatures. This results in a local cooling of the area. Deforestation in thetropics seems to lead to warming as decreased evapotranspiration leads to a decrease inmoisture availability and thus less cloud cover and more incident radiation at thesurface. The two studies discussed in this paper took different approaches to studying theeffects of land cover change on climate. Both produced some interesting results,although more research is still necessary to fully understand the effects of land-coverchange on climate. However, the results of these studies show that it is crucial toinclude these effects on climate when attempting to predict future climates.

References

[1] Brovkin, Victor et. al., (1999), Modelling Climate Response to Historical LandCover Change. Global Ecology & Biogeography, 8, 509-513.

[2] Feddema, Johannes J. et. al., (2005), The Importance of Land-Cover Change inSimulating Future Climates. Science, 310, 1674-1678.

[3] Gibbard, S. et. al., (2005), Climate Effects of Global Land Cover Change. Geophysical Research Letters, 32, 1-4.

[4] Pielke, Roger A. Sr., (2005), Land Use and Climate Change. Science, 310, 1625-1626.

[5] Pielke, Roger A. Sr. et. al., (2002), The Influence of Land-Use Change and Landscape Dynamics on the Climate System: Relevance to Climate-Change Policy beyond the Radiative Effect of Greenhouse Gases. The Royal Society, 360, 1705-1719.

Images

Figure 2. Present day land cover (above) Color legend for land cover types (above right) Projected land cover changes for IPCC SRES B1 and A2 emission scenarios. Hatched regions indicate a removal of agriculture and solid colored regions indicate a change to agriculture. Boxes highlight areas of interest for the study.From Feddema et. al.,

Figure 1. Land cover for the Florida peninsula before 1900 (left) and in the 1990s (right). Highlights the conversion from tree cover (green) to cropland (yellow). From Pielke et. al., 2005

Figure 4. Changes in the annual average diurnal temperature range due to land cover change in the B1 (left) and A2 (right) scenarios. Values were found by subtracting the control run from the run including both land cover change forcing and atmospheric forcing. Shaded grid cells are significant at the 0.05 confidence level. From Feddema et. al., 2005.

Figure 3. JJA temperature differences due to land-cover changes in both the B1 (left) and A2 (right) scenarios for the years 2050 (top) and 2100 (bottom). Shaded grid cells are significant at the 0.05 confidence level. From Feddema et. al., 2005.

Figure 6. Annual mean 2-m air temperature for the tree simulation minus the bare ground simulation (a). Middle latitude tree simulation for 30-50N with current vegetation elsewhere (b). Annual mean surface albedo for the tree simulation minus the bare ground simulation (c). From Gibbard et. al., 2005.

Figure 5. Zonally averaged 2-m air temperature difference for the offline model simulations between different vegetation types and bare ground. From Gibbard et. al., 2005.