Climate change

climate change through efforts like flood control measures or drought-resistant crops partially reduces climate change risks, although some limits to adaptation have already been reached.[14] Poorer communities are responsible for a small share of global emissions, yet have the least ability to adapt and are most vulnerable to climate change.[15][16]

Bobcat Fire in Monrovia, CA, September 10, 2020
Bleached colony of Acropora coral
A dry lakebed in California, which is experiencing its worst megadrought in 1,200 years.[17]
Examples of some effects of climate change: Wildfire intensified by heat and drought, bleaching of corals occurring more often due to marine heatwaves, and worsening droughts compromising water supplies.
Many climate change impacts have been felt in recent years, with 2023 the warmest on record at +1.48 °C (2.66 °F).[18] Additional warming will increase these impacts and can trigger tipping points, such as melting all of the Greenland ice sheet.[19] Under the 2015 Paris Agreement, nations collectively agreed to keep warming "well under 2 °C". However, with pledges made under the Agreement, global warming would still reach about 2.7 °C (4.9 °F) by the end of the century.[20] Limiting warming to 1.5 °C will require halving emissions by 2030 and achieving net-zero emissions by 2050.[21]

Strategies to phase out fossil fuels involve conserving energy, generating electricity cleanly, and using electricity to power transportation, heat buildings, and operate industrial facilities. The electricity supply can be made cleaner and more plentiful by vastly increasing deployment of wind, and solar power, alongside other forms of renewable energy and nuclear power.[22][23] Carbon can also be removed from the atmosphere, for instance by increasing forest cover and farming with methods that capture carbon in soil.[24]Temperature records prior to global warming
Main articles: Climate variability and change; Temperature record of the last 2,000 years; and Paleoclimatology
Prior to human evolution the record includes hotter temperatures and occasional abrupt changes, such as the Paleocene–Eocene Thermal Maximum 55.5 million years ago.[34]

Over the last few million years Human beings evolved in a climate that cycled through ice ages, with global average temperature ranging between current levels and 5–6 °C colder than today.[35][36] Historical patterns of warming and cooling, like the Medieval Warm Period and the Little Ice Age, did not occur at the same time across different regions. Temperatures may have reached as high as those of the late 20th century in a limited set of regions.[37] Climate information for that period comes from climate proxies, such as trees and ice cores.[38]

Warming since the Industrial Revolution
Main article: Instrumental temperature record

In recent decades, new high temperature records have substantially outpaced new low temperature records on a growing portion of Earth's surface.[39]

There has been an increase in ocean heat content during recent decades as the oceans absorb over 90% of the heat from global warming.[40]
Around 1850 thermometer records began to provide global coverage.[41] Between the 18th century and 1970 there was little net warming, as the warming impact of greenhouse gas emissions was offset by cooling from sulfur dioxide emissions. Sulfur dioxide causes acid rain, but it also produces sulfate aerosols in the atmosphere, which reflect sunlight and cause so-called global dimming. After 1970, the increasing accumulation of greenhouse gases and controls on sulfur pollution led to a marked increase in temperature.[42][43][44]Multiple independent datasets all show worldwide increases in surface temperature,[45] at a rate of around 0.2 °C per decade.[46] The 2013-2022 decade warmed to an average 1.15 °C [1.00–1.25 °C] compared to the pre-industrial baseline (1850–1900).[47] Not every single year was warmer than the last: internal climate variability processes can make any year 0.2 °C warmer or colder than the average.[48] From 1998 to 2013, negative phases of two such processes, Pacific Decadal Oscillation (PDO)[49] and Atlantic Multidecadal Oscillation (AMO).[50] caused a so-called "global warming hiatus".[51] After the hiatus, the opposite occurred, with years like 2023 exhibiting temperatures well above even the recent average.[52] This is why the temperature change is defined in terms of a 20-year average, which minimises the noise of hot and cold years and decadal climate patterns, and detects the long-term signal.[53]: 5 [54]

A wide range of other observations reinforce the evidence of warming.[55][56] The upper atmosphere is cooling, because greenhouse gases are trapping heat near the Earth's surface, and so less heat is radiating into space.[57] Warming reduces average snow cover and forces the retreat of glaciers. At the same time, warming also causes greater evaporation from the oceans, leading to more atmospheric humidity, more and heavier precipitation.[58] Plants are flowering earlier in spring, and thousands of animal species have been permanently moving to cooler areas.[59]

Differences by regionDifferent regions of the world warm at different rates. The pattern is independent of where greenhouse gases are emitted, because the gases persist long enough to diffuse across the planet. Since the pre-industrial period, the average surface temperature over land regions has increased almost twice as fast as the global-average surface temperature.[60] This is because oceans lose more heat by evaporation and oceans can store a lot of heat.[61] The thermal energy in the global climate system has grown with only brief pauses since at least 1970, and over 90% of this extra energy has been stored in the ocean.[62][63] The rest has heated the atmosphere, melted ice, and warmed the continents.[64]

The Northern Hemisphere and the North Pole have warmed much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, but also more seasonal snow cover and sea ice. As these surfaces flip from reflecting a lot of light to being dark after the ice has melted, they start absorbing more heat.[65] Local black carbon deposits on snow and ice also contribute to Arctic warming.[66] Arctic surface temperatures are increasing between three and four times faster than in the rest of the world.[67][68][69] Melting of ice sheets near the poles weakens both the Atlantic and the Antarctic limb of thermohaline circulation, which further changes the distribution of heat and precipitation around the globe.[70][71][72][73]Future global temperatures
Further information: Carbon budget and Earth's energy budget

CMIP6 multi-model projections of global surface temperature changes for the year 2090 relative to the 1850–1900 average. The current trajectory for warming by the end of the century is roughly half way between these two extremes.[20][74][75]
The World Meteorological Organization estimates a 66% chance of global temperatures exceeding 1.5 °C warming from the preindustrial baseline for at least one year between 2023 and 2027.[76][77] Because the IPCC uses a 20 year average to define global temperature changes, a single year exceeding 1.5 °C does not break the limit.

The IPCC expects the 20-year average global temperature to exceed +1.5 °C in the early 2030s.[78] The IPCC Sixth Assessment Report (2023) included projections that by 2100 global warming is very likely to reach 1.0-1.8 °C under a scenario with very low emissions of greenhouse gases, 2.1-3.5 °C under an intermediate emissions scenario, or 3.3-5.7 °C under a very high emissions scenario.[79] In the intermediate and high emission scenarios, the warming will continue past 2100.[80][81]

The remaining carbon budget for staying beneath certain temperature increases is determined by modelling the carbon cycle and climate sensitivity to greenhouse gases.[82] According to the IPCC, global warming can be kept below 1.5 °C with a two-thirds chance if emissions after 2018 do not exceed 420 or 570 gigatonnes of CO2. This corresponds to 10 to 13 years of current emissions. There are high uncertainties about the budget. For instance, it may be 100 gigatonnes of CO2 equivalent smaller due to CO2 and methane release from permafrost and wetlands.[83] However, it is clear that fossil fuel resources need to be proactively kept in the ground to prevent substantial warming. Otherwise, their shortages would not occur until the emissions have already locked in significant long-term impacts.[84]Environmental effects
Further information: Effects of climate change on oceans and Effects of climate change on the water cycle
The environmental effects of climate change are broad and far-reaching, affecting oceans, ice, and weather. Changes may occur gradually or rapidly. Evidence for these effects comes from studying climate change in the past, from modelling, and from modern observations.[175] Since the 1950s, droughts and heat waves have appeared simultaneously with increasing frequency.[176] Extremely wet or dry events within the monsoon period have increased in India and East Asia.[177] Monsoonal precipitation over the Northern Hemisphere has increased since 1980.[178] The rainfall rate and intensity of hurricanes and typhoons is likely increasing,[179] and the geographic range likely expanding poleward in response to climate warming.[180] Frequency of tropical cyclones has not increased as a result of climate change.[181]


Historical sea level reconstruction and projections up to 2100 published in 2017 by the U.S. Global Change Research Program[182]
Global sea level is rising as a consequence of thermal expansion and the melting of glaciers and ice sheets. Between 1993 and 2020, the rise increased over time, averaging 3.3 ± 0.3 mm per year.[183] Over the 21st century, the IPCC projects 32–62 cm of sea level rise under a low emission scenario, 44–76 cm under an intermediate one and 65–101 cm under a very high emission scenario.[184] Marine ice sheet instability processes in Antarctica may add substantially to these values,[185] including the possibility of a 2-meter sea level rise by 2100 under high emissions.[186]

Climate change has led to decades of shrinking and thinning of the Arctic sea ice.[187] While ice-free summers are expected to be rare at 1.5 °C degrees of warming, they are set to occur once every three to ten years at a warming level of 2 °C.[188] Higher atmospheric CO2 concentrations cause more CO2 to dissolve in the oceans, which is making them more acidic.[189] Because oxygen is less soluble in warmer water,[190] its concentrations in the ocean are decreasing, and dead zones are expanding.[191