nuclei and lead to clouds that have more and smaller cloud droplets. these clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets. they also reduce the growth of raindrops, which makes clouds more reflective to incoming sunlight. indirect effects of aerosols are the largest uncertainty in radiative forcing. while aerosols typically limit global warming by reflecting sunlight, black carbon in soot that falls on snow or ice can contribute to global warming. not only does this increase the absorption of sunlight, it also increases melting and sea-level rise. limiting new black carbon deposits in the arctic could reduce global warming by 0.2 °c by 2050. land surface changes the rate of global tree cover loss has approximately doubled since 2001, to an annual loss approaching an area the size of italy. humans change the earth's surface mainly to create more agricultural land. today, agriculture takes up 34% of earth's land area, while 26% is forests, and 30% is uninhabitable (glaciers, deserts, etc.). the amount of forested land continues to decrease, which is the main land use change that causes global warming. deforestation releases co2 contained in trees when they are destroyed, plus it prevents those trees from absorbing more co2. the main causes of deforestation are: permanent land-use change from forest to agricultural land producing products such as beef and palm oil (27%), logging to produce forestry/forest products (26%), short term shifting cultivation (24%), and wildfires (23%). the type of vegetation in a region affects the local temperature. it impacts how much of the sunlight gets reflected back into space (albedo), and how much heat is lost by evaporation. for instance, the change from a dark forest to grassland makes the surface lighter, causing it to reflect more sunlight. deforestation can also affect temperatures by modifying the release of chemical compounds that influence clouds, and by changing wind patterns. in tropic and temperate areas the net effect is to produce significant warming, while at latitudes closer to the poles a gain of albedo (as forest is replaced by snow cover) leads to a cooling effect. globally, these effects are estimated to have led to a slight cooling, dominated by an increase in surface albedo. according to fao, forest degradation aggravates the impacts of climate change as it reduces the carbon sequestration abilities of forests. indeed, among their many benefits, forests also have the potential to reduce the impact of high temperatures. solar and volcanic activity as the sun is the earth's primary energy source, changes in incoming sunlight directly affect the climate system. solar irradiance has been measured directly by satellites, and indirect measurements are available from the early 1600s onwards. there has been no upward trend in the amount of the sun's energy reaching the earth. explosive volcanic eruptions represent the largest natural forcing over the industrial era. when the eruption is sufficiently strong (with sulfur dioxide reaching the stratosphere), sunlight can be partially blocked for a couple of years. the temperature signal lasts about twice as long. in the industrial era, volcanic activity has had negligible impacts on global temperature trends. present-day volcanic co2 emissions are equivalent to less than 1% of current anthropogenic co2 emissions. physical climate models are unable to reproduce the rapid warming observed in recent decades when taking into account only variations in solar output and volcanic activity. further evidence for greenhouse gases causing global warming comes from measurements that show a warming of the lower atmosphere (the troposphere), coupled with a cooling of the upper atmosphere (the stratosphere). if solar variations were responsible for the observed warming, the troposphere and stratosphere would both warm. climate change feedback sea ice reflects 50% to 70% of incoming sunlight, while the ocean, being darker, reflects only 6%. as an area of sea ice melts and exposes more ocean, more heat is absorbed by the ocean, raising temperatures that melt still more ice. this process is a positive feedback. the response of the climate system to an initial forcing is modified by feedbacks: increased by "self-reinforcing" or "positive" feedbacks and reduced by "balancing" or "negative" feedbacks. the main reinforcing feedbacks are the water-vapour feedback, the ice–albedo feedback, and the net effect of clouds. the primary balancing mechanism is radiative cooling, as earth's surface gives off more heat to space in response to rising temperature. in addition to temperature feedbacks, there are feedbacks in the carbon cycle, such as the fertilizing effect of co2 on plant growth. uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions. as air warms, it can hold more moisture. water vapour, as a potent greenhouse gas, holds heat in the atmosphere. if cloud cover increases, more sunlight will be reflected back into space, cooling the planet. if clouds become higher and thinner, they act as an insulator, reflecting heat from below back downwards and warming the planet. the effect of clouds is the largest source of feedback uncertainty. another major feedback is the reduction of snow cover and sea ice in the arctic, which reduces the reflectivity of the earth's surface. more of the sun's energy is now absorbed in these regions, contributing to amplification of arctic temperature changes. arctic amplification is also melting permafrost, which releases methane and co2 into the atmosphere. climate change can also cause methane releases from wetlands, marine systems, and freshwater systems. overall, climate feedbacks are expected to become increasingly positive. around half of human-caused co2 emissions have been absorbed by land plants and by the oceans. on land, elevated co2 and an extended growing season have stimulated plant growth. climate change increases droughts and heat waves that inhibit plant growth, which makes it uncertain whether this carbon sink will continue to grow. soils contain large quantities of carbon and may release some when they heat up. as more co2 and heat are absorbed by the ocean, it acidifies, its circulation changes and phytoplankton takes up less carbon, decreasing the rate at which the ocean absorbs atmospheric carbon. overall, at higher co2 concentrations the earth will absorb a reduced fraction of our emissions. modelling projected global surface temperature changes relative to 1850–1900, based on cmip6 multi-model mean changes a climate model is a representation of the physical, chemical, and biological processes that affect the climate system. models also include natural processes like changes in the earth's orbit, historical changes in the sun's activity, and volcanic forcing. models are used to estimate the degree of warming future emissions will cause when accounting for the strength of climate feedbacks, or reproduce and predict the circulation of the oceans, the annual cycle of the seasons, and the flows of carbon between the land surface and the atmosphere. the physical realism of models is tested by examining their ability to simulate contemporary or past climates. past models have underestimated the rate of arctic shrinkage and underestimated the rate of precipitation increase. sea level rise since 1990 was underestimated in older models, but more recent models agree well with observations. the 2017 united states-published national climate assessment notes that "climate models may still be underestimating or missing relevant feedback processes". additionally, climate models may be unable to adequately predict short-term regional climatic shifts. a subset of climate models add societal factors to a simple physical climate model. these models simulate how population, economic growth, and energy use affect – and interact with – the physical climate. with this information, these models can produce scenarios of future greenhouse gas emissions. this is then used as input for physical climate models and carbon cycle models to predict how atmospheric concentrations of greenhouse gases might change. depending on the socioeconomic scenario and the mitigation scenario, models produce atmospheric co2 concentrations that range widely between 380 and 1400 ppm. the ipcc sixth assessment report projects that global warming is very likely to reach 1.0 °c to 1.8 °c by the late 21st century under the very low ghg emissions scenario. in an intermediate scenario global warming would reach 2.1 °c to 3.5 °c, and 3.3 °c to 5.7 °c under the very high ghg emissions scenario. these projections are based on climate models in combination with observations. the remaining carbon budget is determined by modelling the carbon cycle and the climate sensitivity to greenhouse gases. 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 smaller due to methane release from permafrost and wetlands. however, it is clear that fossil fuel resources are too abundant for shortages to be relied on to limit carbon emissions in the 21st century. impacts main article: effects of climate change the sixth ipcc assessment report projects changes in average soil moisture that can disrupt agriculture and ecosystems. a reduction in soil moisture by one standard deviation means that average soil moisture will approximately match the ninth driest year between 1850 and 1900 at that location. environmental effects 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. since the 1950s, droughts and heat waves have appeared simultaneously with increasing frequency. extremely wet or dry events within the monsoon period have increased in india and east asia. the rainfall rate and intensity of hurricanes and typhoons is likely increasing, and the geographic range likely expanding poleward in response to climate warming. frequency of tropical cyclones has not increased as a result of climate change. historical sea level reconstruction and projections up to 2100 published in 2017 by the u.s. global change research program global sea level is rising as a consequence of glacial melt, melt of the greenland ice sheets and antarctica, and thermal expansion. between 1993 and 2020, the rise increased over time, averaging 3.3 ± 0.3 mm per year. over the 21st century, the ipcc projects that in a very high emissions scenario the sea level could rise by 61–110 cm. increased ocean warmth is undermining and threatening to unplug antarctic glacier outlets, risking a large melt of the ice sheet and the possibility of a 2-meter sea level rise by 2100 under high emissions. climate change has led to decades of shrinking and thinning of the arctic sea ice. 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. higher atmospheric co2 concentrations have led to changes in ocean chemistry. an increase in dissolved co2 is causing oceans to acidify. in addition, oxygen levels are decreasing as oxygen is less soluble in warmer water. dead zones in the ocean, regions with very little oxygen, are expanding too. tipping points and long-term impacts greater degrees of global warming increase the risk of passing through ‘tipping points’—thresholds beyond which certain impacts can no longer be avoided even if temperatures are reduced. an example is the collapse of west antarctic and greenland ice sheets, where a temperature rise of 1.5 to 2 °c may commit the ice sheets to melt, although the time scale of melt is uncertain and depends on future warming. some large-scale changes could occur over a short time period, such as a shutdown of certain ocean currents like the atlantic meridional overturning circulation (amoc). tipping points can also include irreversible damage to ecosystems like the amazon rainforest and coral reefs. the long-term effects of climate change on oceans include further ice melt, ocean warming, sea level rise, and ocean acidification. on the timescale of centuries to millennia, the magnitude of climate change will be determined primarily by anthropogenic co2 emissions. this is due to co2's long atmospheric lifetime. oceanic co2 uptake is slow enough that ocean acidification will continue for hundreds to thousands of years. these emissions are estimated to have prolonged the current interglacial period by at least 100,000 years. sea level rise will continue over many centuries, with an estimated rise