There has always been climate change, even before humans existed. A number of different factors contributed to it: Earth’s orbit around the sun, changes in solar activity, volcanic eruptions and many other natural factors. Increased emissions of greenhouse gases are the main cause of the rise in the global mean temperature of the last 50 to 60 years. In the context of the earth's history, natural factors cannot explain the extraordinarily rapid warming in the 20th century.

A major El Niño event at the beginning of the year and very reduced sea ice cover at the north and south poles meant that 2016 held the new record for the warmest year since 1880. The global mean temperature was around 1.1 °C above the pre-industrial level. In Switzerland, 2018 and 2020 were the warmest years on record since 1864 with a deviation of around 1 °C relative to the norm period 1991-2020.

So, why is it so cold on average in Switzerland? This is due to the relatively long winters and long nights. The main determining factor here, however, is the fact that Switzerland has a large proportion of high-altitude regions in the form of the Alps and the Jura region. By way of comparison: the annual mean temperature at the Zurich-Fluntern meteorological station is 9.8 °C, in Lugano 13.0 °C, and on the Jungfraujoch -6.7 °C for the same period. The Swiss mean temperature is a weighted mean from measurements taken at a number of stations. The weighting ensures that the differing levels of altitude are accounted for commensurately.

Carbon dioxide (CO₂) blocks terrestrial out-going thermal radiation, which makes CO₂ a greenhouse gas. Moreover, CO₂ is very long-lived, as it reacts with hardly any other gases in the atmosphere and is also not washed away by rain. These properties make CO₂ the most significant factor in the warming of the earth, which we have now been measuring for some decades. In 2015, humans emitted a total of 36.2 billion tonnes of CO₂. In comparison with the natural outputs of carbon dioxide by oceans, soils, plants and animals, this is a relatively small amount (about 3%). However, the main CO₂ sinks - plants and oceans - are not able to compensate for the anthropogenic excess. The global concentration is therefore rising continually. In 2014, the magic threshold of 400 ppm (0.04%) was exceeded for the first time in millions of years. Also in the year 2020, this trend is ongoing despite the corona virus and associated consequences on mobility and world economy. Prior to industrialisation, the value remained at around 280 ppm over a period of 23 million years. New estimates, which the Intergovernmental Panel on Climate Change (IPCC) rated as reasonably reliable in their latest Assessment Report in 2013, indicate that the figure exceeded 400 ppm for short periods about 800,000 years ago. Continued unchecked carbon emissions would result in today's level being doubled (to around 800 ppm) by the year 2100.

Locally, temperature trends in Switzerland only have very slight variations. But what does a warming of approximately 2°C mean in real terms? The changes can be determined using a wide range of climate indicators: There are, for example, more heat waves and tropical nights today than there were previously. Frost days in low-lying areas have increased in number. What is more, it has been established that the snowline is now higher than it used to be, and the vegetation periods last longer and start earlier.

Over the course of this century, the global temperature is set to rise further. On the basis of an emissions scenario in which fossil fuels continue to be used unabatedly in the future, Switzerland would have to reckon with further warming of 3.3-5.4°C by the year 2100 compared to the mean over the period 1981-2010. Only a very focused programme to reduce global emissions will be able to hold the temperature rise in check at a level of 0.6-1.9°C.

Based on data collected by MeteoSwiss, estimates can be made as to the volume of precipitation that falls on Swiss territory per unit of time. Depending on the year, this is between 50 and 90 cubic kilometres of water. This volume is somewhere between that of Lake Constance and Lake Geneva. This means that the equivalent of a column of water around 1 to 1.8 metres high falls on Swiss territory in a year. And within just one day, the volume of water that falls as precipitation can be the equivalent of the volume contained in Lake Biel. On 7th August 1978 - which had an extremely high level of precipitation - the amount of water contained in Lake Zurich fell from the skies over Switzerland.

Long-term trends in precipitation are superimposed by large variations from year to year, which means that these trends are uncertain at a number of measurement locations. Our measurement series taken over many years in the Swiss plateau point to an increase in the averaged total annual precipitation. This is primarily due to the observed increase in the winter months. Also on the south side of the Alps, there are no significant trends to be detected on the seasonal or annual scale.

The most precipitation in the shortest amount of time - within a few days - falls in Ticino. Heavy precipitation events are actually a summer phenomenon, as the heaviest precipitation tends to occur in connection with thunderstorms. The precipitation intensity of the heaviest precipitation days over the course of a year has seen a slight increase at 90% of Swiss meteorological stations. On average, the increase has been +7.7% per degree of warming, or around 12% since 1901. This trend is expected to continue. It is therefore only a matter of time before Switzerland is inundated in one day with the volume of water contained in Lake Zurich (see climate fact further up this page).

According to the climate change scenarios CH2018, winter precipitation over Switzerland will increase by about +15% (+2 to +24%) with respect to the mean of the period 1981-2010 given an emission path without mitigation. In summer however, mean precipitation is expected to decrease. Under unabated emissions of greenhouse gases, the future summer precipitation could be up to 40% less compared to the mean of 1981-2010 by the end of the century. A swift and global reduction of greenhouse gas emissions could limit those changes markedly.

From observations of when trees come into leaf, for example the horse chestnut (as observed in Geneva) and the cherry tree (in Liestal), the evidence is unmistakeable: the vegetation period has changed. In the 19th century, the first blossoms would appear on the horse chestnut on average not until the beginning of April. Since then, leaves have been emerging earlier and earlier, nowadays often before March, even. Between 1900 and 1960, the cherry tree rarely blossomed before April. Now, early blossoming in March tends to be the norm rather than the exception. There are two main reasons for this. One the one hand, climate change is a factor in the vegetation period starting earlier and ending later. On the other hand, cities have grown considerably since measurements first began. In cities, the temperature is often higher than in the surrounding countryside, as asphalt and buildings heat up to a greater extent during the day, and give off less heat during the night than the natural landscape.

For the above-mentioned events, it is not possible to make predictions about future changes. This is due on the one hand to large degrees of uncertainty in climate models where these events are concerned. On the other hand, the frequency and intensity of such events is already fluctuating widely in today's climate, which means that no long-term trends can be determined.