Climate is undergoing a process of change all around the globe. Convincing and unequivocal evidence for this is provided by a range of indicators. A large proportion of the warming observed since 1850 is attributable to human impacts and, in particular, emissions of greenhouse gases. Future climate scenarios show a continuation of the global rise in temperatures, as well as impacts on many other aspects of the climate. Also Switzerland is affected by these developments. Significant efforts must be made in terms of reducing greenhouse gas emissions if we are to limit the global increase in temperatures in the future.
Global climate change
Switzerland's climate is very varied, and is influenced by the particular characteristics of Alpine topography. At the same time, however, it is part of a supra-regional and global system, and so the climatic developments on a global level also affect climate change in Switzerland.
Global climate change since 1850
The increase in near-surface temperatures since systematic measurements began in 1850 is indisputable, both on the global scale as well as in Switzerland. The global temperature rise is approximately 1°C (Fig. 1). The mean temperatures of the last three decades have been successively higher than those in any of the previous decades since 1850. Moreover, current temperature conditions appear to be extreme even in a long-term context: According to estimates by the Intergovernmental Panel on Climate Change (IPCC) the period 1983-2012 in the northern hemisphere was probably the warmest 30-year period of the last 1,400 years, even accounting for temporarily warmer phases in some parts of the world during the Middle Ages, when temperatures were similar to today's in some cases.
According to consolidated scientific knowledge, the observed temperature rise since the mid-19th century has to a large extent been due to anthropogenic causes. The rise cannot be explained by natural drivers alone, such as variations in solar activity or volcanic eruptions (Figure 2). There is a clear-cut causal relationship between the warming and the increased concentrations of greenhouse gases in the atmosphere, and particularly a rise in the concentration levels of carbon dioxide (CO2) as a result of the use of fossil fuels. Figure 3 shows the rising trend in atmospheric CO2 concentrations since 1958, from measurements taken at the observatory on the Mauna Loa volcano in Hawaii. A distinct seasonal pattern that occurs primarily as a result of the annual vegetation cycle is superimposed by a strong positive, long-term trend. Because of the remoteness of the observatory, the lack of local influencing factors, and the generally good mixing of CO2 in the lower atmosphere, these measurements are a good reflection of global CO2 concentrations. Consequently, today's level stands at around 400 ppm (parts per million - in other words, one molecule of CO2 per million total molecules of dry air) in contrast to the level of around 320 ppm at the start of the 1960s. This concentration seems little enough, however CO2, along with other trace gases, is a very effective greenhouse gas, with a significant impact on the earth's radiation budget. Reconstructions from ice cores verify that the greenhouse gas concentration levels over the past 800,000 years have never been as high as they are now, and that current levels are at an absolute maximum for this period (IPCC Fifth Assessment Report).
The rise in near-surface global mean temperature described here is not the only consequence of increased greenhouse gas concentrations. There are a number of other parameters that have a direct or indirect relationship with global mean temperature. Figure 4 shows a selection of these. In addition to an unequivocal increase in near-surface air temperature above land and see, a significant increase is also observed in air temperature averaged over the whole lower atmosphere (the troposphere). The same is true for sea surface temperature, oceanic heat content and the global mean sea level. Snow and ice are inextricably linked to the temperature rise and are seeing a marked decline. Considered together, these and other indicators leave no room for doubt that we are now in a period of significant and rapid global warming.
What about the future?
We can rely on observations and reconstructions to analyse climate change in the past. For the future, however, such information is obviously not available, and a simple extrapolation of past trends is problematical in a system with changing framework conditions. So how do we arrive at possible future climate scenarios?
This is where simulations using climate models come in. Similar to the weather forecast, these comprehensive computer models which are based on physical laws describe processes in the atmosphere and in other components of the climate system, as well as their interactions.
If these models are checked against observed climate change, they can be utilised for climate projections, for the purposes of which assumptions are made on the future developments in terms of global greenhouse gas emissions. Model simulations thus demonstrate the effect of increasing greenhouse gas concentrations on a wide range of parameters, including global mean temperature, precipitation and global sea level.
Often, an ensemble of different climate models is run using the same emission scenario, leading to different results depending on the particular model used, which allows the scientists to evaluate the uncertainties involved.
The projected global temperature rise by the year 2100 is shown in Figure 5, together with the range from the different models used (shaded areas in the upper panel and bars on the right). The strong dependency of temperature rise on future greenhouse gas emissions can be seen clearly from this illustration (red in comparison to blue). The drastic scenario of RCP8.5 (continued unmitigated increase in greenhouse gas emissions) results in a mean temperature increase of approximately 4°C. The RCP2.6 scenario, which assumes imminent, radical measures to reduce global emissions, limits the temperature increase to around 1°C from today's levels. When the warming to date since the mid-19th century is taken into consideration, this remains within the much-discussed 2-degrees warming target. The related projections for global sea level rises (lower panel) range from around +0.4 m (mean value for RCP2.6) to +0.6 m (mean value for RCP8.5).
Similarly to the trends observed to date, future climate change will also take on different characteristics from region to region. This means, therefore, that the globally averaged indicators of change are not applicable to all regions of the earth. Figure 6 shows the spatial distribution of predicted temperature and precipitation changes by the end of the 21st century, again for the emission scenarios RCP2.6 (left) and RCP8.5 (right). A relatively high degree of warming is expected to occur at high latitudes, primarily as a result of the decline in sea ice and seasonally snow-covered areas (polar amplification). Models are in good agreement as far as this pattern is concerned (dotted areas). Areas of land, particularly those in the continental interiors, generally warm to a greater extent than the thermally inert oceans. Both emission scenarios show qualitatively similar spatial distributions, albeit at differing levels. While temperature increases of more than 7°C are expected for the drastic RCP8.5 scenario, the projected rise in temperature for RCP2.6 is less than 2°C for large expanses of the earth's surface.
In the extreme scenario of RCP8.5, the spatial pattern of change in average annual precipitation indicates significant increases at high latitudes as well as, with some exceptions, along the equator, whereas reductions are expected in many subtropical regions and in the Mediterranean. In general, there are much greater discrepancies between models in relation to the spatial pattern for precipitation change than for temperature (large cross-hatched areas). For RCP2.6 the projected changes are small. For the extreme RCP8.5 scenario, however, changes can extend either side of -30% and +50%.