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What we currently know about global climate change

Climate change is a global phenomenon with consequences for all regions of the Earth. According to the findings of the UN Intergovernmental Panel on Climate Change (IPCC), it has been established that the global warming observed to date is mainly attributable to human emissions of greenhouse gases. Climate scenarios for the future indicate that global temperatures will continue to rise and impact many other systems such as the water cycle, ice masses and ecological systems, with the extent of the impact being strongly contingent on future greenhouse gas emissions.

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The IPCC (Intergovernmental Panel on Climate Change) compiles a comprehensive status report approximately every seven years, detailing the latest knowledge on climate change. This page outlines the key findings of the Sixth Physical Science Basis Report, published in 2021.

History of climate change since 1850

Climate change is already having an impact today, and the warming is mainly due to human emissions of greenhouse gases.

The effects of climate change are already discernible

The mean global temperature has increased by approximately 1.1°C since industrialisation began, with greater warming seen over land (+1.6°C) than over the oceans (+0.9°C). This rate of warming is unprecedented in the last 2,000 years. The consequences have now become evident and verifiable in every corner of the globe. Other impacts of climate change include rising sea levels, melting ice masses, thawing permafrost, and shrinking snow cover. Many of the climate and weather extremes that have occurred in recent years are partly the result of climate change. There has been a measurable increase in hot-temperature extremes in almost every region of the Earth, and many regions have also seen increases in precipitation extremes or drought.

Human influence on climate change has been proven beyond doubt

It has been unequivocally proven that humans are the primary contributors to global warming. According to the IPCC, almost every observed instance of warming can be attributed to human activity. Purely natural factors (such as solar activity, volcanic eruptions or changes in the Earth’s orbit) are not sufficient to explain the increase in the mean global temperature, which is clearly and causally related to increasing concentrations of greenhouse gases in the atmosphere, and, in particular, of carbon dioxide concentrations due to human use of fossil fuels.

The CO2 concentration measured in the atmosphere is currently the highest it has been for at least two million years. In addition to CO2, other greenhouse gases that contribute to climate change include methane (CH4) and nitrous oxide (N2O). The warming caused by greenhouse gases is only partially mitigated by the cooling effect of aerosols.

What does natural variability mean in terms of climate?

Natural variability refers to climatic fluctuations which are caused by processes that occur without any human influence. These include natural external factors such as fluctuating solar activity or changes in the Earth’s orbit, as well as fluctuations determined by the chaotic nature of our complex climate system. Natural fluctuations are one of the main causes of global climatic changes from year to year, and can mask the human-induced warming trend for several years or even decades. However, the influence of natural fluctuations is generally small in the context of trends over several decades or longer. Over the historical period of 1850–2020, natural variability made little to no contribution to global surface warming (range -0.23 °C to +0.23 °C). This is negligible compared to the observed warming of around 1.1°C in the same period, which is almost exclusively attributable to human influence.

Future climate change

The climate will continue to warm over the coming years and decades, with far-reaching consequences. Climatic extremes such as heat, heavy precipitation and drought will become even more extreme as warming continues.

Temperatures will continue to rise

The mean global temperature will also continue to rise in the future. A warming of 1.5°C above pre-industrial levels can be expected by 2030. How temperatures develop thereafter will depend heavily on decisions made today on whether and to what degree human-induced greenhouse gas emissions are reduced over the next few years. Restricting global warming to below 2°C will require a significant reduction in CO2 and other greenhouse gases (climate targets). Temperature rises in the future will not be the same in every region of the world, but will follow a certain pattern, whereby the warming is projected to be more pronounced over land than over the oceans, and especially marked over the polar regions. Switzerland will also experience greater temperature increases than the global mean.

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 the models reliably reproduce the observed climate trends, they can be deployed for climate projections. The process also involves various assumptions on the future trend of global greenhouse gas emissions. Model simulations thus illustrate the impacts of increasing greenhouse gas concentrations on a wide range of parameters, including global mean temperature, precipitation conditions and global sea level. Generally speaking, an ensemble, or range, of different climate models are run for the same emissions assumption, which produce different results depending on the model used. This allows the overall ensemble evaluation to take account of the inherent uncertainties.

Other consequences of continued warming

Global warming has an impact on many other components of the climate system. Mean precipitation is expected to increase at high latitudes, near to the equator and in some monsoon regions, and to decrease in other areas, especially in the subtropics. Similar changes are projected in terms of soil moisture, with slight variations due to local evaporation. Other consequences of continued climate change are further reductions in Arctic sea ice, snow cover and permafrost. Rising sea levels, ocean acidification and changes in ocean currents can also be expected.

In addition, ocean and land carbon sinks will become less efficient as greenhouse gas emissions continue – in other words, they will not be able to absorb as much CO2 as they do today. This is likely to lead to a larger proportion of CO2 emissions remaining in the atmosphere. Furthermore, a situation in which feedback processes add further additional greenhouse gases to the atmosphere cannot be ruled out. An example of this is the increased emission of the greenhouse gas methane from land sinks in the event of permafrost thawing. Climate change not only has an impact on the climate system, but also on many other areas, including the biosphere, agriculture, health and the economy.

Climate extremes are projected to occur more frequently and with greater intensity

Continued global warming will mean further changes in terms of extreme events. For example, each additional half-degree rise in global temperature is expected to lead to a significant increase in the intensity and frequency of extreme heat and heavy precipitation events. Some regions will experience more frequent and extreme phases of drought. If global warming continues, concurrent extreme events – an unlikely occurrence in the past and in today’s climate – will also become more frequent.

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