November 22, 2008

Why is Land Use/Land Cover Change a First-Order Climate Forcing?

Roger Pielke Sr.
ClimateSci.org
Originally posted on August 5, 2005.

As recognized by the National Research Council in 2005, land-use/land-cover change is a first-order climate forcing. However, its role as a regional and global climate influence is not widely recognized, except as it effects the atmospheric concentration of carbon dioxide and the global average surface albedo. In the summary figure from the IPCC Statement for Policymakers (see Figure ES-2 here), in terms of the global mean radiative forcing, only albedo effects of land use/land cover change are identified.

However, numerous studies have shown that the effect of land-cover/land-use change is to alter temperatures and precipitation in regions where the change occurs, as well as weather globally through teleconnections (see, for example, 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 and The climatic impacts of land-surface change and carbon management, and the implications for climate change mitigation policy).

The reason for this influence is described in a presentation I gave entitled "Land-Use/Land-Cover Change as a Major Climate Forcing: Evidence and Consequences for Climate Research." In the talk, I asked the question "why should landscape effects, which cover only a fraction of the Earth's surface, have global circulation effects?" The answer can be summarized as follows:

1. Land-use/land-cover change alters the surface fluxes of heat and water vapor from what they were before the change. This alteration in the fluxes affects the atmospheric boundary layer, and the energy available for thunderstorms.
2. As shown in pioneering work by Joanne Simpson and Herbert Riehl, globally from 1500-5000 thunderstorms (which are referred to as "hot towers") are the conduit to transport heat, moisture and wind energy to higher latitudes. Since thunderstorms occur only in a relatively small percentage of the Earth's surface, a change in their spatial patterns would be expected to have global consequences.
3. Most thunderstorms (by a ratio of about 10 to 1) occur over land.
4. The regional alteration in tropospheric diabatic heating has a large influence on the climate system (see my July 28th blog).
5. Global climate effects occur with ENSO events since they are of large magnitude, have long persistence, and are spatially coherent. Regional land-use/land-cover changes have the same and larger spatial scales (see Australian Land Clearing, A Global Perspective: Latest Facts & Figures for changes in landscape in the 1990s). Regional land-use/land-cover changes have a large magnitude, long persistence, and are spatially coherent.

We should, therefore expect global climate effects from land-use/land-cover change. The next IPCC needs to focus more on this first-order climate forcing than they have in the past. The question of searching for a "discernable effect on the climate system" misses the obvious in that we have been altering regional and global climate by land-use/land-cover change for decades. The goal of "preventing dangerous anthropogenic interference with the climate system" (from the UN Framework Convention on Climate Change, article 2, 1999), by focusing on CO₂, has overlooked the first order climate forcing of land-use/land-cover change in altering the surface heat and water vapor fluxes.
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