[16] Hernandez et al. [2006] performed a comprehensive evaluation of how well MM5 simulates the climate of Central America. They focused on all available daily station observations and found that, at least where these relatively sparse observations exist, the model does an acceptable job in describing the regional climate. In particular, they found that model simulated temperature, wind speed, and vapor mixing ratio agreed well with the observations. Precipitation, especially its magnitude, was handled less well overall, although this was attributed mainly to the lack of stations where sharp topographic features occur. The spatial patterns of precipitation showed much closer agreement. Because of the sparse network of reporting stations, especially in regions of complex topography, we also compared precipitation from the MM5 control run to precipitation obtained from the Tropical Rainfall Measurement Mission (TRMM). We used the TRMM6 blended product ( http://trmm.gsfc.nasa.gov/ ); this combines data from TRMM itself as well as all other available, quality‐checked precipitation data. This analysis provided little additional insight to that of Hernandez et al., suggesting that the spatial patterns were reasonably well simulated but the magnitude was more problematic. Other, quasi‐proxy observations such as global reanalyses are not helpful because of their coarse spatial resolution (typically 1.0°–2.5° in latitude and longitude) and lack of assimilated station data for this region. Summarizing, while the lack of sufficient observations makes it difficult to fully assess the performance of MM5, what evidence we do have suggests that the model does at least a credible job.

3.2. Deforestation Scenarios

[17] The potential climatic impacts of deforestation are best demonstrated by the full deforestation versus completely forested cases (Figure 2). Replacing trees with grassland has two major effects: (1) an increase in surface albedo, which leads to cooling and stabilization of the atmosphere [Charney, 1975], and (2) a large reduction in evapotranspiration from the surface, leading to warming and stabilization of the atmosphere [Henderson et al., 1993; Suh and Lee, 2004]. This occurs because the energy no longer used for evapotranspiration goes into heating the surface. The warmer surface then warms the air above, leading to rising parcels of air. These parcels of air, by themselves, stabilize the atmosphere by leading to higher pressure aloft (the “thermal mountain” effect of Stern and Malkus [1953]). (Destabilization is caused by large‐scale changes in the environmental or background, vertical temperature structure of the atmosphere, not by rising parcels per se.) Both processes (1) and (2) therefore tend to stabilize the atmosphere and reduce precipitation [Charney, 1975; Oglesby and Erickson, 1989].

Figure 2 Open in figure viewer PowerPoint Surface temperature differences (in °C) for the MM5 simulation with all grassland minus all forested MM5 runs for (a) January and (b) July. Precipitation differences (in cm) for (c) January and (d) July.

[18] To clarify their relative effects, we made an additional run in which only the albedo differences between forest and grassland were imposed. We found that the albedo increase led to a general 1°C–2°C cooling. Furthermore, while many previous deforestation studies have been made, they generally focused on large landmasses (e.g., the Amazon or tropical Africa). A key purpose of our study is to see how deforestation might affect a region that is much smaller and closer to large water bodies (oceans). Other more minor effects also occur, such as a reduction in surface roughness when grassland replaces forest, which affects winds, and also inhibits evaporative fluxes from the surface. While small, these act to enhance the overall effects of deforestation.

[19] Warmer and drier conditions occur in both the dry (Figures 2a and 2b) and wet (Figures 2c and 2d) seasons, but, not surprisingly, the impact is much larger in the wet season for both temperature and rainfall. That is, during the dry season, precipitation and surface evaporation are both at relative minimums regardless of the nature of the land cover. Temperatures warm by 1°C–2°C during the dry season, and precipitation generally shows a modest decrease. During the wet season, on the other hand, temperatures warm by up to 3°C–5°C, and precipitation is reduced by up to 15%–30%. Temperatures increase everywhere (except for a small region around Lake Nicaragua). Approximately 78% of the overall Maya region (southwest Mexico, Guatemala, southern Yucatan, and Honduras) shows a precipitation decrease. Relative to the mean, precipitation decreases an average of 17% throughout this region, with maximum decreases of 29%. Unfortunately, even 5 year model simulations are insufficient for robust statistical significance testing, but a 5 year 17% decrease would generally be considered at least drought‐like (see, for example, http://www.ndmc.unl.edu).

[20] These changes are largest where the landmasses are largest and smallest over Panama and Costa Rica, where landmasses are smaller and nowhere far from the ocean. Furthermore, in general, the precipitation decrease is largest where the temperature increase is also largest. The precipitation increases are smaller, and indeed in some places increase, where the increase in temperature is also smaller. This is consistent with the physical effects of deforestation described above; we would expect them to vary simultaneously, and indeed, this is what occurs. Physically, the larger the landmass, the larger the change in forestation and, importantly, the smaller the ameliorating effects of adjacent oceans. This also means that the largest changes in both temperature and precipitation generally coincide with those regions most populated by the Maya. The reduction in rainfall means it would have been more difficult for the Maya to store enough water to survive the dry season, while the warmer temperatures would increase evaporation stress, as well as stress agriculture, livestock, and people.

[21] While precipitation decreases strongly over land, it also shows an overall increase over the adjacent oceans. When averaged over the domain as a whole, the net precipitation change is quite small. While the cause is not certain, this is likely a question of compensation. As noted by Rogers [1988] and Magana et al. [1999], the wet season for much of Central America crucially depends on convection that originates over adjacent ocean waters and then drifts over land. The increase in surface heating due to deforestation in general tends to block convection [e.g., Stern and Malkus, 1953]. Therefore, the most likely reason for the increased oceanic precipitation is simply that the convective storms are inhibited from moving over land and instead remain over the ocean.

[22] Therefore, our results are best considered a sensitivity study that sets overall limits on the role of deforestation in affecting Mesoamerica climate, with implications for the collapse of Maya civilization. Certainly, the complete deforestation case may be a valid model for the latter stages of the classic Maya civilization, at least in those regions they occupied, but even before the earliest human settlements, it is likely that not all of Mesoamerica was completely forested. Furthermore, as noted by Pielke [2001], a patchwork of forest and grassland may affect mesoscale circulations and hence have an effect on convective activity. To gain a more modern‐day perspective, we compared temperature and precipitation for land use patterns in 2000 versus circa 1980 (Figure 3) and complete deforestation except for the MBC versus circa 1980 (Figure 4). Noteworthy in Figures 3 and 4 is that the changes are very small. Indeed, the changes between 1980 and 2000 are almost trivially small, while even those between 1980 and the MBC case are hard to distinguish from year‐to‐year variations within each simulation. Consideration of the land use plots shown in Figure 1 leads to the realization that by 1980 much of Mesoamerica had already been deforested, thus subsequent loss of forest would have only a small impact on climate. This result is consistent with anecdotal evidence that, by the 1940s and 1950s, extensive deforestation had already taken place. What small changes in precipitation do take place have the same general spatial pattern as in the all grassland minus all forested case. This helps support the robustness of our overall results; we also see little impact of the patchwork forest effects of Pielke [2001] although our 12 km resolution may be insufficient to fully account for them.

Figure 3 Open in figure viewer PowerPoint Differences for July for the run with 2000 land use types minus the “control” (circa 1980): (a) surface temperature differences (in °C) and (b) precipitation differences (in cm).

Figure 4 Open in figure viewer PowerPoint Differences for July for the run with proposed MBC land use types versus the “control” (circa 1980): (a) surface temperature differences (in °C) and (b) precipitation differences (in cm).

[23] As a sobering aside, this result also implies that modern‐day Mesoamerica is already experiencing the warming and drying that deforestation engenders. This further suggests that it may not be sufficient for the political entities that comprise Mesoamerica to simply discontinue deforestation; regions previously deforested must be replanted with new forest. Unfortunately, we do not know the pristine pre‐Maya state of Mesoamerican forestation; we can only conjecture that it must have been much more extensive than the known circa 1980 forest extent.

[24] Figure 5 shows the possibility of natural drought in these regions where the Maya civilization flourished. Figure 5a indicates the area over which precipitation was averaged over the Mayan lowland region of Guatemala and the Yucatan. Shown in Figures 5b and 5c are time series of warm season rainfall (defined as rain from 1 June through 30 September). Figure 5b is from the 100 year “present‐day” CCSM3 control run, that is, with constant boundary conditions and forcings, which allows an evaluation of natural climatic variability. In particular, this run had climatological SST prescribed, such that each model year had the same annual cycle of SST. Numerous dry periods are evident, although only a few extend for more than a few years. Figure 5c is from the simulation run for the period 1870–1999. This run prescribed observed year‐to‐year variations SST. Again, numerous dry periods are evident, but the key difference is that some prolonged periods of dry conditions occur, including one period of 24 of 30 years from 1952 through 1981. The obvious conclusion is twofold: (1) Large year‐to‐year variations in precipitation can occur even in the absence of any specific change in forcing or especially SST. (2) When specific, known climatic forcings are used in the model, much more extensive periods of drought can occur. While further analyses are required to substantiate, the most likely implication is that specific SST patterns that persist for some time can lead to prolonged drought. Many studies have documented this for other regions, especially North America [e.g., Feng et al., 2008, and references therein]. Overall, it is clear that naturally occurring drought undoubtedly plays a key role in this area.