Over the last three decades of the 20th century, GDP per capita and population growth were

the main drivers of increases in greenhouse gas emissions. burning of fossil fuels and land-use change.
[43]

CO2 emissions are continuing to rise due to the

[44][45]:71

Emissions scenarios, estimates of changes in future

[46]

emission levels of greenhouse gases, have been projected that depend upon uncertain economic, sociological, technological, and natural developments. In most scenarios, emissions
[47][48]

continue to rise over the century, while in a few, emissions are reduced.

These emission scenarios,

combined with carbon cycle modelling, have been used to produce estimates of how atmospheric concentrations of greenhouse gases will change in the future. Using the six IPCC SRES"marker" scenarios, models suggest that by the year 2100, the atmospheric concentration of CO 2could range between 541 and 970 ppm.
[49]

This is an increase of 90-250% above the concentration in the year 1750.

[50]

Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if coal, tar sands or methane clathrates are extensively exploited.

The destruction of stratospheric ozone by chlorofluorocarbons is sometimes mentioned in relation to global warming. Although there are a few areas of linkage, the relationship between the two is not strong. Reduction of stratospheric ozone has a cooling influence. until the late 1970s.
[52] [53] [51]

Substantial ozone depletion did not occur

Ozone in the troposphere (the lowest part of the Earth's atmosphere) does

contribute to surface warming.

1.2 Aerosols and soot

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, has partially counteracted global warming from 1960 to the present.
[54]

The main cause of this dimming

is aerosols produced by volcanoes and pollutants. These aerosols exert a cooling effect by increasing the reflection of incoming sunlight. The effects of the products of fossil fuel combustionCO2 and aerosols have largely offset one another in recent decades, so that net warming has been due to the increase in non-CO2 greenhouse gases such as methane.
[55]

Radiative forcing due to aerosols is temporally limited

due to wet deposition which causes aerosols to have an atmospheric lifetime of one week. Carbon dioxide has a lifetime of a century or more, and as such, changes in aerosol concentrations will only delay climate changes due to carbon dioxide.
[57] [56]

In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget. Sulfate aerosols act as cloud condensation nuclei and thus lead to clouds that
[58]

have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets. This effect also causes droplets to be of more uniform size, which
[59]

reduces growth of raindrops and makes the cloud more reflective to incoming sunlight.

Indirect effects
[60]

are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Aerosols, particularly their indirect effects, represent the largest uncertainty in radiative forcing.

Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric soot aerosols directly absorb solar radiation, which heats the atmosphere and cools the surface. In isolated areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.
[61]

Atmospheric soot

always contributes additional warming to the climate system. When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.
[62]

The influences of
[63]

aerosols, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extra tropics and southern hemisphere.

1.3 Solar variation

Variations in solar output have been the cause of past climate changes.
[65] [31][66][67][68] [64]

The effect of changes in solar

forcing in recent decades is uncertain, but small, with some studies showing a slight cooling effect, while others studies suggest a slight warming effect.

Greenhouse gases and solar forcing affect temperatures in different ways. While both increased solar activity and increased greenhouse gases are expected to warm the troposphere, an increase in solar activity should warm the stratosphere while an increase in greenhouse gases should cool the stratosphere.
[31]

Observations show that temperatures in the stratosphere have been cooling since 1979,
[69]

when satellite measurements became available. Radiosonde (weather balloon) data from the pre-satellite era show cooling since 1958, though there is greater uncertainty in the early radiosonde record.

A related hypothesis, proposed by Henrik Svensmark, is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate. rays.
[70]

Other research has found no relation between warming in recent decades and cosmic The influence of cosmic rays on cloud cover is about a factor of 100 lower than needed to

[71][72]

explain the observed changes in clouds or to be a significant contributor to present-day climate change.

2. Attributed and expected effects

2.1 Natural systems
Global warming has been detected in a number of systems. Some of these changes, e.g., based on the instrumental temperature record, have been described in the section on temperature changes. Rising sea levels and observed decreases in snow and ice extent are consistent with warming.
[86] [17]

Most of the
[D]

increase in global average temperature since the mid-20th century is, with high probability, to human-induced changes in greenhouse gas concentrations.
[87]

attributable

Even with current policies to reduce emissions, global emissions are still expected to continue to grow over the coming decades. Over the course of the 21st century, increases in emissions at or above their

current rate would very likely induce changes in the climate system larger than those observed in the 20th century. In the IPCC Fourth Assessment Report, across a range of future emission scenarios, model-based estimates of sea level rise for the end of the 21st century (the year 2090-2099, relative to 1980-1999) range from 0.18 to 0.59 m. These estimates, however, were not given likelihood due to a lack of scientific understanding, nor were an upper bound given for sea level rise. Over the course of centuries to millennia, the melting of ice sheets could result in sea level rise of 46 m or more.
[88]

Changes in regional climate are expected to include greater warming over land, with most warming at high northern latitudes, and least warming over the Southern Ocean and parts of the North Atlantic Ocean.
[87]

Snow cover area and sea ice extent are expected to decrease. The frequency of hot extremes,

heat waves, and heavy precipitation will very likely increase.

2.2 Ecological systems

In terrestrial ecosystems, the earlier timing of spring events, and pole ward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming.
[17]

Future climate change is

[87]

expected to particularly affect certain ecosystems, including tundra, mangroves, and coral reefs.
[89]

It is

expected that most ecosystems will be affected by higher atmospheric CO 2 levels, combined with higher global temperatures. Overall, it is expected that climate change will result in the extinction of many
[90]

species and reduced diversity of ecosystems.

2.3 Social systems

There is some evidence of regional climate change affecting systems related to human activities, including agricultural and forestry management activities at higher latitudes in the Northern Hemisphere.
[17]

Future climate change is expected to particularly affect some sectors and systems related
[87]

to human activities.

Low-lying coastal systems are vulnerable to sea level rise and storm surge. Human

health will be at increased risk in populations with limited capacity to adapt to climate change. It is expected that some regions will be particularly affected by climate change, including the Arctic, Africa, small islands, and Asian and African mega deltas. In some areas the effects on agriculture, industry and health could be mixed, or even beneficial in certain respects, but overall it is expected that these benefits will be outweighed by negative effects.
[91]

3. Responses to global warming

3.1 Mitigation
Reducing the amount of future climate change is called mitigation of climate change. The IPCC defines mitigation as activities that reduce greenhouse gas (GHG) emissions, or enhance the capacity of carbon sinks to absorb GHGs from the atmosphere.
[92]

Many countries, both developing and developed, are

aiming to use cleaner, less polluting, technologies.

[45]:192

Use of these technologies aids mitigation and

could result in substantial reductions in CO2 emissions. Policies include targets for emissions reductions, increased use of renewable energy, and increased energy efficiency. Studies indicate substantial potential for future reductions in emissions.
[93]

Since even in the most optimistic scenario, fossil fuels are

going to be used for years to come, mitigation may also involve carbon capture and storage, a process that traps CO2produced by factories and gas or coal power stations and then stores it, usually underground.
[94]

3.2 Adaptation
Other policy responses include adaptation to climate change. Adaptation to climate change may be planned, e.g., by local or national government, or spontaneous, i.e., done privately without government intervention.
[95] [93]

The ability to adapt is closely linked to social and economic development.

Even

societies with high capacities to adapt are still vulnerable to climate change. Planned adaptation is already occurring on a limited basis. The barriers, limits, and costs of future adaptation are not fully understood. Another policy response is engineering of the climate (geo engineering). This policy response is sometimes grouped together with mitigation. estimates for it have not yet been published.
[96]

Geo engineering is largely unproven, and reliable cost

[97]

3.3 UNFCCC
Most countries are Parties to the United Nations Framework Convention on Climate Change (UNFCCC).
[98]

The ultimate objective of the Convention is to prevent "dangerous" human

[99]

interference of the climate system.

As is stated in the Convention, this requires that GHGs are

stabilized in the atmosphere at a level where ecosystems can adapt naturally to climate change, food production is not threatened, and economic development can proceed in a sustainable fashion. The UNFCCC recognizes differences among countries in their responsibility to act on climate change.
[100]

In the Kyoto Protocol to the UNFCCC, most developed countries (listed in Annex I of the
[101]

treaty) took on legally binding commitments to reduce their emissions. response to these commitments have reduced emissions. countries, reducing poverty is their overriding aim.
th [104] [103] [102]

Policy measures taken in

For many developing (non-Annex I)

At the 15 UNFCCC Conference of the Parties, held in 2009 at Copenhagen, several UNFCCC Parties produced the Copenhagen Accord. Parties agreeing with the Accord aim to limit the future increase in
[105]

global mean temperature to below 2 C.