Changes in historic climate due to changes in land-use

What was the vegetation like 200 years ago? Did humans effect the climate before emitting CO2 by changing the vegetation? Rossby Centre is involved in a project dealing with reconstructions of past vegetation and the coupling between vegetation and climate in historic times.

Long before humans started to emit CO2 from fossil fuel we had an influence on the climate when cutting down trees to make way for graze land and agricultural land. Deforestation effects the climate in two different ways, the global climate changes because of the increased amounts of CO2 in the atmosphere from cutting down and burning wood, and the local climate changes when the land surface properties change due to changed vegetation.

Previously, the effects from increased atmospheric CO2 from changed vegetation and land-use over the last 8000 years have been discussed (Ruddiman, 2005) and the direct effects of changed vegetation have been studied on a global scale (e.g. Brovkin et al., 2006; Pitman et al., 2009). A new project has studied the direct effect from changed vegetation on a regional scale in Europe. The project includes simulations of past climate and past vegetation and also assumptions of historic land-use.

Using climate models to reconstruct historic vegetation

The Rossby Centre regional climate model (RCA3) has been run for two different time periods, 6 000 years ago and 200 years ago. For each period RCA3 was first run with modern day vegetation, the simulated climate was then used in a dynamical vegetation model, LPJ-GUESS, to produce potential vegetation that is consistent with the simulated climate. With this vegetation as a starting point, assumptions of historic land-use are applied to produce estimates of the vegetation.

RCA3 is run with three different vegetation, potential vegetation and two different land-use assumptions (Hyde (Klein Goldewijk et al, 2001) and Kaplan (Kaplan et al., 2009)). When trying to reconstruct historic vegetation (e.g. from pollen in lake sediments) information on how changes in vegetation and land-use effects climate is of great value when evaluating the methods of reconstruction.

Deforestation affects the temperature climate

In RCA3 changes in vegetation is mainly expressed as changes in the fraction of forest. When it comes to potential vegetation the forest is able to grow freely giving a large fraction of forest, while in the land-use estimations the forest is to a certain extent cut down giving a lower fraction of forest, i.e. more open land (Fig. 1).


Fig 1 Gustav RC news 2012 april
Figure 1. Fraction of forest (%) in the three different vegetation estimates at 200 year before present. Potential vegetation (left), potential vegetation with land-use from Hyde (middle) and potential vegetation with land-use from (Kaplan).
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Deforestation mainly effects climate in two ways: by changes in albedo and by changes in evaporation and soil water. Albedo is a surface's ability to reflect solar radiation, open land reflects more solar radiation than trees and is therefore colder. This effect is enhanced in winter since open land more readily gets covered by snow and thereby reflecting even more solar radiation. At the same time deforestation reduces the efficiency of transpiration; giving lower latent heat flux and higher temperature. In some regions this effect exceeds the albedo effect.

The model results show that the simulations including land-use are colder in winter with 0-1.5 °C. In summer the picture is more complex, some areas get 0-1.5 °C colder and some 0-1.0 °C warmer (Fig. 2).

Fig 2 Gustav RC news 2012 april
Figure 2. Mean temperature (°C) in January (top line) and July (bottom line) at 200 years before present. Left: absolute temperature for potential vegetation (PV) simulation. Middle: difference in temperature between PV and PVHyde (PVHyde – PV). Right: difference in temperature between PV and PVKaplan (PVKaplan – PV).
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The project is a collaboration between researchers from Rossby Centre, Linneaus University, Lund University and Tallin University.

References

Brovkin, V., Claussen, M., Dreisschaert, E., Fichefet, T., Kicklighter, D., Loutre, M. F., Matthews, H. D., Ramankutty, N., Schaeffer, M. and Sokolov, A.: Biogeophysical effects of historical land cover changes simulated by six Earth system models of intermediate complexity, Clim. Dyn., 26, 587-600, 2006.

Kaplan, J., Krumhardt, K., and Zimmerman, N.: The prehistoric and preindustrial deforestation of Europe, Quarternary Sci. Rev., 28, 3016-3034, 2009.

Klein Goldewijk, K.: Estimating global land use change over the past 300 years: The HYDE database, Global Biogeochem. Cy., 15, 417-433, 2001.

Pitman, A. J., de Noblet-Ducoudré, N., Cruz, F. T., Davin, E. L., Bonan, G. B., Brovkin, V., Claussen, M., Delire, C., Ganzeveld, L., Gayler, V., van den Hurk, B. J. J. M., Lawrence, P. J., van der Molen, M. K., Müller, C., Reick, C. H., Seneviratne, S. I., Strengers, B. J. and Voldoire, A.: Uncertainties in climate responses to past land cover changes: First results from the LUCID intercomparison study, Geophys. Res. Lett., 36, L14814, 2009.

Ruddiman, W. F.: How did humans first alter global climate?, Scientific American, March 2005, 2005.