Observed temperature changes

Global warming

Выполнил учащийся 1 курса группы 78А отделения «Агрономия»

Брель Константин

Contents:

1.Introduction
2 Observed temperature changes
2.1 Trends
2.2 Warmest years
3. Initial causes of temperature changes (external forcings)
3.1 Greenhouse gases
3.2 Aerosols and soot
3.3 Solar activity
3.4 Variations in Earth's orbit
4. Feedback
5. Climate models
6. Observed and expected environmental effects
6.1 Extreme weather
6.2 Sea level rise
6.3 Ecological systems
6.4 Long-term effects
6.5 Large-scale and abrupt impacts
7. Observed and expected effects on social systems
7.1 Habitat inundation
7.2 Economy
7.3 Infrastructure
8. Possible responses to global warming
8.1 Mitigation
8.2 Adaptation
8.3 Climate engineering
9. Discourse about global warming
9.1 Political discussion
9.2 Scientific discussion
9.3 Discussion by the public and in popular media
9.3.1 Surveys of public opinion
10. Etymology

Observed temperature changes

Main article: Instrumental temperature record

2015 – Warmest Global Year on Record (since 1880) – Colors indicate temperature anomalies (NASA/NOAA; 20 January 2016).^[37]

Earth has been in radiative imbalance since at least the 1970s, where less energy leaves the atmosphere than enters it. Most of this extra energy has been absorbed by the oceans.^[38] It is very likely that human activities substantially contributed to this increase in ocean heat content.^[39]

Two millennia of mean surface temperatures according to different reconstructions from climate proxies, each smoothed on a decadal scale, with the instrumental temperature record overlaid in black.

NOAA graph of Global Annual Temperature Anomalies 1950–2012, showing the El Niño Southern Oscillation

The global average (land and ocean) surface temperature shows a warming of 0.85 [0.65 to 1.06] °C in the period 1880 to 2012, based on multiple independently produced datasets.^[40] Earth's average surface temperature rose by 0.74±0.18 °C over the period 1906–2005. The rate of warming almost doubled for the last half of that period (0.13±0.03 °C per decade, versus 0.07±0.02 °C per decade).^[41]

The average temperature of the lower troposphere has increased between 0.13 and 0.22 °C (0.23 and 0.40 °F) per decade since 1979, according to satellite temperature measurements. Climate proxies show the temperature to have been relatively stable over the one or two thousand yearsbefore 1850, with regionally varying fluctuations such as the Medieval Warm Period and the Little Ice Age.^[42]

The warming that is evident in the instrumental temperature record is consistent with a wide range of observations, as documented by many independent scientific groups.^[43] Examples include sea level rise,^[44] widespread melting of snow and land ice,^[45] increased heat content of the oceans,^[43] increased humidity,^[43] and the earlier timing of spring events,^[46] e.g., the flowering of plants.^[47] The probability that these changes could have occurred by chance is virtually zero.^[43]

Trends

Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).^[48] Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.^[49] Since the beginning of industrialisation the temperature difference between the hemispheres has increased due to melting of sea ice and snow in the North.^[50] Average arctic temperatures have been increasing at almost twice the rate of the rest of the world in the past 100 years; however arctic temperatures are also highly variable.^[51] Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.^[52]

The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at year 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.

Global temperature is subject to short-term fluctuations that overlay long-term trends and can temporarily mask them. The relative stability in surface temperature from 2002 to 2009, which has been dubbed the global warming hiatus by the media and some scientists,^[54] is consistent with such an episode.^[55][56] 2015 updates to account for differing methods of measuring ocean surface temperature measurements show a positive trend over the recent decade.

Warmest years

15 of the top 16 warmest years have occurred since 2000.^[59] While record-breaking years can attract considerable public interest, individual years are less significant than the overall trend. So some climatologists have criticized the attention that the popular press gives to "warmest year" statistics; for example, Gavin Schmidt stated "the long-term trends or the expected sequence of records are far more important than whether any single year is a record or not.

2015 was not only the warmest year on record, it broke the record by the largest margin by which the record has been broken. 2015 was the 39th consecutive year with above-average temperatures. Ocean oscillations like El Niño Southern Oscillation (ENSO) can affect global average temperatures, for example, 1998 temperatures were significantly enhanced by strong El Niño conditions. 1998 remained the warmest year until 2005 and 2010 and the temperature of both of these years was enhanced by El Niño periods. The large margin by which 2015 is the warmest year is also attributed to another strong El Niño. However, 2014 was ENSO neutral. According to NOAA and NASA, 2015 had the warmest respective months on record for 10 out of the 12 months. The average temperature around the globe was 1.62˚F (0.90˚C) or 20% above the twentieth century average. In a first, December 2015 was also the first month to ever reach a temperature 2 degrees Fahrenheit above normal for the planet.

Initial causes of temperature changes (external forcings)

Main article: Attribution of recent climate change

Greenhouse effect schematic showing energy flows between space, the atmosphere, and Earth's surface. Energy exchanges are expressed in watts per square meter (W/m²).

This graph, known as the Keeling Curve, documents the increase of atmospheric carbon dioxideconcentrations from 1958–2015. Monthly CO₂ measurements display seasonal oscillations in an upward trend; each year's maximum occurs during the Northern Hemisphere's late spring, and declines during its growing season as plants remove some atmospheric CO₂.

The climate system can warm or cool in response to changes in external forcings. ^[63][64] These are "external" to the climate system but not necessarily external to Earth.^[65]Examples of external forcings include changes in atmospheric composition (e.g., increased concentrations of greenhouse gases), solar luminosity, volcanic eruptions, andvariations in Earth's orbit around the Sun.^[66]

Greenhouse gases

Main articles: Greenhouse gas, Greenhouse effect, Radiative forcing, Carbon dioxide in Earth's atmosphere and Earth's energy budget

See also: List of countries by carbon dioxide emissions and History of climate change science

The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in a planet's atmosphere warm its lower atmosphere and surface. It was proposed by Joseph Fourier in 1824, discovered in 1860 by John Tyndall,^[67] was first investigated quantitatively by Svante Arrhenius in 1896,^[68] and was developed in the 1930s through 1960s by Guy Stewart Callendar.

Annual world greenhouse gas emissions, in 2010, by sector.

Percentage share of global cumulative energy-related CO₂ emissions between 1751 and 2012 across different regions.

On Earth, naturally occurring amounts of greenhouse gases have a mean warming effect of about 33 °C (59 °F).^[70][d] Without the Earth's atmosphere, the Earth's average temperature would be well below the freezing temperature of water.^[71] The major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect;carbon dioxide (CO₂), which causes 9–26%; methane (CH₄), which causes 4–9%; and ozone (O₃), which causes 3–7%.^[72][73][74] Clouds also affect the radiation balance through cloud forcings similar to greenhouse gases.

Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO₂, methane, tropospheric ozone, CFCs and nitrous oxide. According to work published in 2007, theconcentrations of CO₂ and methane have increased by 36% and 148% respectively since 1750.^[75] These levels are much higher than at any time during the last 800,000 years, the period for which reliable data has been extracted from ice cores.^{[76][77][78][79]} Less direct geological evidence indicates that CO₂ values higher than this were last seen about 20 million years ago.

Fossil fuel burning has produced about three-quarters of the increase in CO₂ from human activity over the past 20 years. The rest of this increase is caused mostly by changes in land-use, particularly deforestation.^[81] Estimates of global CO₂ emissions in 2011 from fossil fuel combustion, including cement production and gas flaring, was 34.8 billion tonnes (9.5 ± 0.5 PgC), an increase of 54% above emissions in 1990. Coal burning was responsible for 43% of the total emissions, oil 34%, gas 18%, cement 4.9% and gas flaring 0.7%

Atmospheric CO₂ concentration from 650,000 years ago to near present, using ice core proxy data and direct measurements.

In May 2013, it was reported that readings for CO₂ taken at the world's primary benchmark site in Mauna Loa surpassed 400 ppm. According to professor Brian Hoskins, this is likely the first time CO₂ levels have been this high for about 4.5 million years. Monthly global CO₂ concentrations exceeded 400 ppm in March 2015, probably for the first time in several million years. On 12 November 2015, NASA scientists reported that human-made carbon dioxide continues to increase above levels not seen in hundreds of thousands of years: currently, about half of the carbon dioxide released from the burning of fossil fuels is not absorbed by vegetation and the oceans and remains in the atmosphere.

Over the last three decades of the twentieth century, gross domestic product per capita and population growth were the main drivers of increases in greenhouse gas emissions. CO₂ emissions are continuing to rise due to the burning of fossil fuels and land-use change. Emissions can be attributed to different regions. Attributions of emissions due to land-use change are subject to considerable uncertainty.

Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, have been projected that depend upon uncertain economic, sociological, technological, and natural developments. In most scenarios, emissions continue to rise over the century, while in a few, emissions are reduced. Fossil fuel reserves are abundant, and will not limit carbon emissions in the 21st century. Emission scenarios, combined with modelling of the carbon cycle, have been used to produce estimates of how atmospheric concentrations of greenhouse gases might change in the future. Using the six IPCC SRES "marker" scenarios, models suggest that by the year 2100, the atmospheric concentration of CO₂ could range between 541 and 970 ppm. This is 90–250% above the concentration in the year 1750.

The popular media and the public often confuse global warming with ozone depletion, i.e., the destruction of stratospheric ozone (e.g., the ozone layer) bychlorofluorocarbons. Although there are a few areas of linkage, the relationship between the two is not strong. Reduced stratospheric ozone has had a slight cooling influence on surface temperatures, while increased tropospheric ozone has had a somewhat larger warming effect.

Aerosols and soot

Ship tracks can be seen as lines in these clouds over the Atlantic Ocean on the east coast of the United States. Atmospheric particles from these and other sources could have a large effect on climate through the aerosol indirect effect.

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, was observed from 1961 until at least 1990. Solid and liquid particlesknown as aerosols, produced by volcanoes and human-made pollutants, are thought to be the main cause of this dimming. They exert a cooling effect by increasing the reflection of incoming sunlight. The effects of the products of fossil fuel combustion – CO₂ and aerosols – have partially offset one another in recent decades, so that net warming has been due to the increase in non-CO₂ greenhouse gases such as methane. Radiative forcing due to aerosols is temporally limited due to the processes that remove aerosols from the atmosphere. Removal by clouds and precipitation gives tropospheric aerosols an atmospheric lifetime of only about a week, while stratospheric aerosols can remain for a few years. Carbon dioxide has a lifetime of a century or more, and as such, changes in aerosols will only delay climate changes due to carbon dioxide.^[101] Black carbon is second only to carbon dioxide for its contribution to global warming.^[102]

In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the Earth's radiation budget. Sulfate aerosols act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets, a phenomenon known as the Twomey effect.^[103]This effect also causes droplets to be of more uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming sunlight, known as the Albrecht effect.^[104] Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Indirect effects of aerosols represent the largest uncertainty in radiative forcing.^[105]

Soot may either cool or warm Earth's climate system, depending on whether it is airborne or deposited. Atmospheric soot directly absorbs 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.^[106] When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.^[107]The influences of atmospheric particles, 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 extratropics and southern hemisphere.

Changes in Total Solar Irradiance(TSI) and monthly sunspot numbers since the mid-1970s.

Contribution of natural factors and human activities to radiative forcing of climate change.^[109] Radiative forcing values are for the year 2005, relative to the pre-industrial era (1750). The contribution of solar irradiance to radiative forcing is 5% the value of the combined radiative forcing due to increases in the atmospheric concentrations of carbon dioxide,methane and nitrous oxide.

Solar activity

Main article: Solar activity and climate

Since 1978, solar irradiance has been measured by satellites.^[111] These measurements indicate that the Sun's radiative output has not increased since 1978, so the warming during the past 30 years cannot be attributed to an increase in solar energy reaching the Earth.

Climate models have been used to examine the role of the Sun in recent climate change.^[112] Models are unable to reproduce the rapid warming observed in recent decades when they only take into account variations in solar output and volcanic activity. Models are, however, able to simulate the observed 20th century changes in temperature when they include all of the most important external forcings, including human influences and natural forcings.

Another line of evidence against solar variations having caused recent climate change comes from looking at how temperatures at different levels in the Earth's atmosphere have changed.^[113] Models and observations show that greenhouse warming results in warming of the lower atmosphere (the troposphere) but cooling of the upper atmosphere (the stratosphere).^[114][115] Depletion of the ozone layer by chemical refrigerants has also resulted in a strong cooling effect in the stratosphere. If solar variations were responsible for observed warming, warming of both the troposphere and stratosphere would be expected.