Effects of climate change are well documented and growing for Earth's natural environment and human societies. Changes to the climate system include an overall warming trend, changes to precipitation patterns, and more extreme weather. As the climate changes it impacts the natural environment with effects such as more intense forest fires, thawing permafrost, and desertification. These changes impact ecosystems and societies, and can become irreversible once tipping points are crossed. Climate activists are engaged in a range of activities around the world that seek to ameliorate these issues or prevent them from happening.

The effects of climate change vary in timing and location. Up until now the Arctic has warmed faster than most other regions due to climate change feedbacks. Surface air temperatures over land have also increased at about twice the rate they do over the ocean, causing intense heat waves. These temperatures would stabilize if greenhouse gas emissions were brought under control. Ice sheets and oceans absorb the vast majority of excess heat in the atmosphere, delaying effects there but causing them to accelerate and then continue after surface temperatures stabilize. Sea level rise is a particular long term concern as a result. The effects of ocean warming also include marine heatwaves, ocean stratification, deoxygenation, and changes to ocean currents. The ocean is also acidifying as it absorbs carbon dioxide from the atmosphere.

The ecosystems most immediately threatened by climate change are in the mountains, coral reefs, and the Arctic. Excess heat is causing environmental changes in those locations that exceed the ability of animals to adapt. Species are escaping heat by migrating towards the poles and to higher ground when they can. Sea level rise threatens coastal wetlands with flooding. Decreases in soil moisture in certain locations can cause desertification and damage ecosystems like the Amazon rainforest.IPCC, 2019: Summary for Policymakers. In: Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems [P.R. Shukla, J. Skea, E. Calvo Buendia, V. Masson-Delmotte, H.- O. Pörtner, D. C. Roberts, P. Zhai, R. Slade, S. Connors, R. van Diemen, M. Ferrat, E. Haughey, S. Luz, S. Neogi, M. Pathak, J. Petzold, J. Portugal Pereira, P. Vyas, E. Huntley, K. Kissick, M. Belkacemi, J. Malley, (eds.)]. At of warming, around 10% of species on land would become critically endangered.

Humans are vulnerable to climate change in many ways. Sources of food and fresh water can be threatened by environmental changes. Human health can be impacted by weather extremes or by ripple effects like the spread of infectious diseases. Economic impacts include changes to agriculture, fisheries, and forestry. Higher temperatures will increasingly prevent outdoor labor in tropical latitudes due to heat stress. Island nations and coastal cities may be inundated by rising sea levels. Some groups of people may be particularly at risk from climate change, such as the poor, children, and indigenous peoples. Industrialised countries, which have emitted the vast majority of CO2, have more resources to adapt to global warming than developing nations do. Cumulative effects and extreme weather events can lead to displacement and migration.

Changes in temperature

Global warming affects all parts of Earth's climate system. Global surface temperatures have risen by . Scientists say they will rise further in the future. The changes in climate are not uniform across the Earth. In particular, most land areas have warmed faster than most ocean areas. The Arctic is warming faster than most other regions. Night-time temperatures have increased faster than daytime temperatures. The impact on nature and people depends on how much more the Earth warms.Schneider, S.H., S. Semenov, A. Patwardhan, I. Burton, C.H.D. Magadza, M. Oppenheimer, A.B. Pittock, A. Rahman, J.B. Smith, A. Suarez and F. Yamin, 2007: Chapter 19: Assessing key vulnerabilities and the risk from climate change. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 779-810.

Scientists use several methods to predict the effects of human-caused climate change. One is to investigate past natural changes in climate. To assess changes in Earth's past climate scientists have studied tree rings, ice cores, corals, and ocean and lake sediments. These show that recent temperatures have surpassed anything in the last 2,000 years.The 20th century was the hottest in nearly 2,000 years, studies show , 25 July 2019 By the end of the 21st century, temperatures may increase to a level last seen in the mid-Pliocene. This was around 3 million years ago.Nicholls, R.J., P.P. Wong, V.R. Burkett, J.O. Codignotto, J.E. Hay, R.F. McLean, S. Ragoonaden and C.D. Woodroffe, 2007: Chapter 6: Coastal systems and low-lying areas. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 315-356. At that time, mean global temperatures were about warmer than pre-industrial temperatures. The global mean sea level was up to higher than it is today.Oppenheimer, M., B.C. Glavovic, J. Hinkel, R. van de Wal, A.K. Magnan, A. Abd-Elgawad, R. Cai, M. Cifuentes-Jara, R.M. DeConto, T. Ghosh, J. Hay, F. Isla, B. Marzeion, B. Meyssignac, and Z. Sebesvari, 2019: Chapter 4: Sea Level Rise and Implications for Low-Lying Islands, Coasts and Communities. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 321–445. . The modern observed rise in temperature and concentrations has been rapid. Even abrupt geophysical events in Earth's history do not approach current rates.Allen, M.R., O.P. Dube, W. Solecki, F. Aragón-Durand, W. Cramer, S. Humphreys, M. Kainuma, J. Kala, N. Mahowald, Y. Mulugetta, R. Perez, M.Wairiu, and K. Zickfeld, 2018: Chapter 1: Framing and Context. In: Global Warming of 1.5 °C. An IPCC Special Report on the impacts of global warming of 1.5 °C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 49-92. .

How much the world warms depends on human greenhouse gas emissions and on how sensitive the climate is to greenhouse gases. The more carbon dioxide () is emitted in the 21st century the hotter the world will be by 2100. For a doubling of greenhouse gas concentrations, the global mean temperature would rise by about . If emissions of stopped abruptly and there was no use of negative emission technologies, the Earth's climate would not start moving back to its pre-industrial state. Temperatures would stay at the same high level for several centuries. After about a thousand years, 20% to 30% of human-emitted would remain in the atmosphere. The ocean and land would not have taken them. This would commit the climate to a warmer state long after emissions have stopped.

With current mitigation policies the temperature will be about 2.7 °C (2.0–3.6 °C) above pre-industrial levels by 2100. It would rise by if governments achieved all their unconditional pledges and targets. If all the countries that have set or are considering net-zero targets achieve them, the temperature will rise by around . There is a big gap between national plans and commitments and the actions that governments have taken around the world.

In September 2025, The German Physical society and the German meteorological society published a joint press release, stating that if the current trends will continue, global temperature can rise to 3 °C above pre-industrial level by the year 2050, and by 5 °C by the year 2100.

Weather

The lower and middle atmosphere, where nearly all weather occurs, are heating due to the greenhouse effect. Evaporation and atmospheric moisture content increase as temperatures rise. Water vapour is a greenhouse gas, so this process is a self-reinforcing feedback.

The excess water vapour also gets caught up in storms. This makes them more intense, larger, and potentially longer-lasting. This in turn causes rain and snow events to become stronger and leads to increased risk of flooding. Extra drying worsens natural dry spells and droughts. This increases risk of heat waves and wildfires. Scientists have identified human activities as the cause of recent climate trends. They are now able to estimate the impact of climate change on extreme weather events using a process called extreme event attribution. For instance such research can look at historical data for a region and conclude that a specific heat wave was more intense due to climate change. In addition, the time shifts of the season onsets, changes in the length of the season durations have been reported in many regions of the world.Schwartz, M.D. and Reiter, B.E. (2000) Changes in North American spring. International Journal of Climatology, 20, 929–932.Hekmatzadeh, A.A., Kaboli, S. and Torabi Haghighi, A. (2020) New indices for assessing changes in seasons and in timing characteristics of air temperature. Theoretical and Applied Climatology, 140, 1247–1261. .Kozlov, M.V. and Berlina, N.G. (2002) Decline in the length of the summer season on the Kola Peninsula, Russia. Climatic Change, 54, 387–398Sparks, T.H. and Menzel, A. (2002) Observed changes in seasons: an overview. International Journal of Climatology, 22, 1715–1725.Aksu, H. (2022). A determination of season shifting across Turkey in the period 1965–2020. International Journal of Climatology, 42(16), 8232–8247. As a result of changes in climatic patterns and rising global temperatures, extreme weather events like heatwaves and heavy precipitation are occurring more frequently and with increasing severity.

Heat waves and temperature extremes

Heatwaves over land have become more frequent and more intense in almost all world regions since the 1950s, due to climate change. Heat waves are more likely to occur simultaneously with droughts. Marine heatwaves are twice as likely as they were in 1980. Climate change will lead to more very hot days and fewer very cold days.IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US. There are fewer cold waves.