The global climate is currently warming and this trend is expected to continue towards an even warmer world, associated partly with drastic shifts in precipitation regimes. While the global temperature has roughly been warming by 0.6°C (±0.2°C) during the 20th century, the land masses have had a higher temperature increase during the same period, and some areas such as the Alps showed an exceptionally high warming trend, with temperature increases of 1.7°C in some regions. Here we report on the current state of the art in climate model projections for the Alps, with an outlook to the soon-to-be-available 5th IPCC assessment report.
It is a challenging task to project what the climate may look like in 50-100 years, a time period relevant to forest management planning cycles. Climatologists use a range of models to generate possible climate futures. Each model and each simulation run is considered a representation of what the climate development over the 21st century might look like. For forest management and decision-making, we have to accept that no exact forecast is possible. Rather, we have to base our planning on the projected trends including their uncertainty. At a global scale, the periodic reports by the Intergovernmental Panel on Climate Change (IPCC) summarize the state of the art of how scientists see the development of the future climate and the associated impacts on ecosystems, the economy and society.
As the 5th assessment report is being finalized, some comparisons to the last two reports are already possible. The 4th assessment report provided a narrower range of for the likely future of the global climate stating that temperatures will likely be between 2.0 and 4.5°C warmer than the period from 1961-1990 (with 66% likelihood), and it also stated that temperature increases of more than 4.5°C are also within the realm of possibility; however, the most likely temperature increase will be 3.0°C by 2100. First indications from global climate modelling studies for the 5th IPCC assessment report project an increase of 2.4-4.9°C as medians from three different scenarios of radiative forcing (following different emission scenarios that are similar to those used in earlier reports).
A fourth scenario is added that assumes a more rigorous and rapid reduction of greenhouse gases than was ever used before, predicting a median temperature increase of only 1.1°C during the 21st Century. Overall, the model simulations for the 5th IPCC assessment report anticipate a 14% likelihood of a global temperature increase exceeding 4.9°C, but that the most likely warming scenario at the global scale is still 3.0°C. Thus, in general, the newest scenarios do project similar warming trends as the 4th IPCC assessment report, although some scenarios point to somewhat higher warming trends than those calculated for the 4th report.
The global climate is simulated using so-called general circulation models (GCMs), which project the future climate based on physics-based processes and first-principles. For regional applications, such model outputs are not very useful, as the spatial resolution of GCMs is very coarse, usually in the range of 1°-2.5° Lat/Lon per model cell. Thus, for regions like the Alps only very few cells are modelled and no terrain elevation is considered. In order to obtain more realistic climate projections at a regional to local scale, two types of downscaling are often combined. First, so-called regional climate models (RCM) are applied to certain larger regions of the World (such as all or part of Europe). These models contain the same mechanisms as the GCMs, are fed by GCM output, and then simulate the climate evolution within the study region by using the input of the GCMs from outside the study region.
The output of these models is thus very similar, providing a range of climate variables at high temporal and moderate spatial resolution, ranging typically between 15-50 km per cell. This is a much better spatial representation of the climate in regions and the output is somewhat sensitive to mountains and their effects on the climate system, though often the output is still too coarse for management and decision-making. Therefore, a further statistics-based downscaling procedure is applied in order to scale the output from RCMs to finer spatial resolution ranging from 100m to 1km, which can be considered well-suited for management applications.
For the MANFRED project, we have used five different RCMs driven by four different GCMs resulting in six GCM/RCM combinations in order to study the impact of likely climate changes on forest species and ecosystems. Table 1 gives an overview of the models used, which originate mostly from the ENSEMBLES EU project, using GCM runs that were calculated for the 4th IPCC assessment report.
We downscaled basic RCM output variables such as monthly temperature and precipitation to finer spatial resolution, typically to 1km or 100m cell size. We are only interested in projecting the relative difference between simulated recent past and simulated futures. Once anomalies are generated, we interpolate these anomalies onto the high resolution of existing climate maps such as Worldclim and add them to these maps to project the future climate changes to the representations of the existing climate.
Figure 1 illustrates the projected climate change trend from the six RCM simulations used, showing examples of the annual and seasonal (summer and winter half) means along with the uncertainty in projected summer climates.
For temperature, we observe a general warming trend in the range of 1.8 to 3.8 °C, with least warming in the winter half, and highest warming in the summer months. The Alps generally face higher warming trends than the surrounding mainland, specifically in the winter months. In summer, the warming is more pronounced in the Western Alps, and seen over the Southern Alps, while the northern ranges and lowlands face a lower degree of warming. Uncertainty among the six models is highest in the higher altitudes of the Alps, and generally increases towards the eastern Alps. For precipitation, the annual trend is not very strong, with some regions south of the Alps experiencing an increase in precipitation, while most of the regions will experience slightly less precipitation. However, the seasonal differences are large.
The summer half of the year is projected to experience significantly less precipitation, with some regions in the Central Alps experiencing only 70% of current summer rainfall rate, while some regions to the Southwest and to the East of the Alps are projected to experience similar precipitation rates as those of today. The winter half of the year is projected to be wetter for most regions, especially the Southwestern Alps, with the Po plain and some Mediterranean regions obtaining less rainfall than today (+20%). The uncertainty among models is highest along the Mediterranean coast and is also relatively high in the Po plain, while in the Alps and in the plains north of the Alps, the six models show comparably high agreement.
A significant change can also be expected from a change in seasonality (Fig. 2). The CLM model projects a "Mediteranization" of the climate, by projecting significantly lower levels of summer precipitation than today, and by simulating increased spring (March, April) and autumn (November) rainfall compared to today, and notably so after ca. 2050.
For forest management this means management taking into account warmer and drier summers, which will have significant effects on some tree species, notably those with lower drought tolerance. This trend is particularly strong throughout the southern of the Alps.