Lecture 13: IPCC Summary For PolicyMakers: WG1-Science

Intergovernmental Panel on Climate Change

Main Conclusions of the SPM

IPCC Web Site

http://www.ipcc.ch

Major Conclusions in SPM

An increasing body of observations gives a collective picture of a warming world and other changes in the climate system.

Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate.

Confidence in the ability of models to project future climate has increased.

There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.

Human influences will continue to change atmospheric composition throughout the 21st century.

Global average temperature and sea level are projected to rise under all IPCC SRES scenarios.

Anthropogenic climate change will persist for many centuries.

Further action is required to address remaining gaps in information and understanding.

Evidences of Global Warming and Other Changes

Temperature

Precipitation

Snow / Ice Cover

Sea Level

Circulation

Extremes

The Land and Oceans Have Warmed

Global Surface Temperature

Compared to the Past 1000 Years

Global Sea Level Has Risen

Precipitation Patterns Have Changed

Other Climate Changes

The snow cover has decreased by about 10% since the late 1960s.

Northern hemisphere spring and summer sea-ice extent has decreased by about 10 to 15% since the 1950s.

El Nino phenomenon has been more frequent, persistent, and intense since the mid-1970s.

No significant trends of Antarctic sea-ice extent are apparent since 1978.

No systematic changes in the frequency of tornadoes, thunder days, or hail events are evident in the limited areas analyzed.

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Slide 17

Emissions of Greenhouse Gases and Aerosols

Concentrations of atmospheric greenhouse gases and their radiative forcing have continued to increase as a result of human activities.

Anthropogenic aerosols are short-lived and mostly produce negative radiative forcing.

Radiative Forcing

Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system, and is an index of the importance of the factor as a potential climate change mechanism. It is expressed in Watts per square metre (Wm^-2).

Slide 20

Increase of CO₂ Concentration

The atmospheric concentration of carbon dioxide (CO2) has increased by 31% since 1750. The present CO2 concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The current rate of increase is unprecedented during at least the past 20,000 years.

About three-quarters of the anthropogenic emissions of CO2 to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is predominantly due to land-use change, especially deforestation.

Currently the ocean and the land together are taking up about half of the anthropogenic CO2 emissions. On land, the uptake of anthropogenic CO2 very likely exceeded the release of CO2 by deforestation during the 1990s.

Slide 22

Increase of N₂O Concentration

The atmospheric concentration of nitrous oxide (N2O) has increased by 46 ppb (17%) since 1750 and continues to increase. The present N2O concentration has not been exceeded during at least the past thousand years.

About a third of current N2O emissions are anthropogenic (e.g., agricultural soils, cattle feed lots and chemical industry).

Increase of Methane Concentration

The atmospheric concentration of methane (CH4) has increased by 151% (1060 ppb9) since 1750 and continues to increase.

The present CH4 concentration has not been exceeded during the past 420,000 years.

Slightly more than half of current CH4 emissions are anthropogenic (e.g., use of fossil fuels, cattle, rice agriculture and landfills).

Halocarbon Concentration

Since 1995, the atmospheric concentrations of many of those halocarbon gases that are both ozone-depleting and greenhouse gases (e.g., CFCl3 and CF2Cl2), are either increasing more slowly or decreasing, both in response to reduced emissions under the regulations of the Montreal Protocol and its Amendments.

Their substitute compounds (e.g., CHF2Cl and CF3CH2F) and some other synthetic compounds (e.g., perfluorocarbons (PFCs) and sulphur hexafluoride (SF6)) are also greenhouse gases, and their concentrations are currently increasing.

Ozone Concentration

The observed depletion of the stratospheric ozone (O3) layer from 1979 to 2000 is estimated to have caused a negative radiative forcing (–0.15 Wm^-²).

The total amount of O3 in the troposphere is estimated to have increased by 36% since 1750, due primarily to anthropogenic emissions of several O3-forming gases. This corresponds to a positive radiative forcing of 0.35 Wm^-².

Anthropogenic Aerosols

The major sources of anthropogenic aerosols are fossil fuel and biomass burning. These sources are also linked to degradation of air quality and acid deposition.

In general, the direct radiative forcing of aerosols is negative (except for black carbon fossil).

There is much less confidence in the ability to quantify the total aerosol direct effect, and its evolution over time.

Aerosols also vary considerably by region and respond quickly to changes in emissions.

In addition to their direct radiative forcing, aerosols have an indirect radiative forcing through their effects on clouds.

There is now more evidence for this indirect effect, which is negative, although of very uncertain magnitude.

Radiative Forcing

Natural Factors of Radiative Forcing

Since the late 1970s, satellite instruments have observed small oscillations due to the 11-year solar cycle. Mechanisms for the amplification of solar effects on climate have been proposed, but currently lack a rigorous theoretical or observational basis.

Stratospheric aerosols from explosive volcanic eruptions lead to negative forcing, which lasts a few years. Several major eruptions occurred in the periods 1880 to 1920 and 1960 to 1991.

The combined change in radiative forcing of the two major natural factors (solar variation and volcanic aerosols) is estimated to be negative for the past two, and possibly the past four, decades.

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Slide 31

More Confidence in Climate Models

Evidence of Global Warming

Detection and attribution studies consistently find evidence for an anthropogenic signal in the climate record of the last 35 to 50 years.

Simulations of the response to natural forcings alone (i.e., the response to variability in solar irradiance and volcanic eruptions) do not explain the warming in the second half of the 20th century.

However, they indicate that natural forcings may have contributed to the observed warming in the first half of the 20th century.

Most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.

Slide 34

IPCC Special Report on Emission Scenarios (SRES)

Projected CO2 Emissions

Projected CO2 Concentration

Projected Aerosol (SO2) Emissions

Projected Global Temperature

The globally averaged surface temperature is projected to increase by 1.4 to 5.8°C over the period 1990 to 2100.

The projected warming is very likely to be without precedent during at least the last 10,000 years….

Global Distribution of The Warming

Projected Precipitation

Based on global model simulations and for a wide range of scenarios, global average water vapor concentration and precipitation are projected to increase during the 21^st century.

By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter.

At low latitudes there are both regional increases and decreases over land areas.

Larger year to year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected.

Projected Extreme Events

Projected El Nino Activity

Current projections show little change or a small increase in amplitude for El Niño events over the next 100 years.

Even with little or no change in El Niño amplitude, global warming is likely to lead to greater extremes of drying and heavy rainfall and increase the risk of droughts and floods that occur with El Niño events in many different regions.

Projected Monsoon Variability

It is likely that warming associated with increasing greenhouse gas concentrations will cause an increase of Asian summer monsoon precipitation variability.

Changes in monsoon mean duration and strength depend on the details of emission scenario.

Projected Thermohaline Circulation

Most models show weakening of the ocean thermohaline circulation which leads to a reduction of the heat transport into high latitudes of the Northern Hemisphere.

The current projections using climate models do not exhibit a complete shut-down of the thermohaline circulation by 2100.

Beyond 2100, the thermohaline circulation could completely, and possibly irreversibly, shut-down in either hemisphere if the change in radiative forcing is large enough and applied long enough. scenarios.

Projected Snow/Ice Cover

Northern Hemisphere snow cover and sea-ice extent are projected to decrease further.

Glaciers and ice caps are projected to continue their widespread retreat during the 21st century.

The Antarctic ice sheet is likely to gain mass because of greater precipitation, while the Greenland ice sheet is likely to lose mass because the increase in runoff will exceed the precipitation increase.

Projected Sea-Level Change

Global mean sea level is projected to rise by 0.09 to 0.88 metres between 1990 and 2100, for the full range of SRES scenarios.

Sea Level Change and Global Warming

Global Warming and Sea-Level Change


	Intergovernmental Panel on Climate Change
	Main Conclusions of the SPM


	An increasing body of observations gives a collective picture of a warming world and other changes in the climate system.
	Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate.
	Confidence in the ability of models to project future climate has increased.
	There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.
	Human influences will continue to change atmospheric composition throughout the 21st century.
	Global average temperature and sea level are projected to rise under all IPCC SRES scenarios.
	Anthropogenic climate change will persist for many centuries.
	Further action is required to address remaining gaps in information and understanding.


	Temperature
	Precipitation
	Snow / Ice Cover
	Sea Level
	Circulation
	Extremes


	The snow cover has decreased by about 10% since the late 1960s.
	Northern hemisphere spring and summer sea-ice extent has decreased by about 10 to 15% since the 1950s.
	El Nino phenomenon has been more frequent, persistent, and intense since the mid-1970s.
	No significant trends of Antarctic sea-ice extent are apparent since 1978.
	No systematic changes in the frequency of tornadoes, thunder days, or hail events are evident in the limited areas analyzed.


	Concentrations of atmospheric greenhouse gases and their radiative forcing have continued to increase as a result of human activities.
	Anthropogenic aerosols are short-lived and mostly produce negative radiative forcing.


	Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system, and is an index of the importance of the factor as a potential climate change mechanism. It is expressed in Watts per square metre (Wm^-2).


	The atmospheric concentration of carbon dioxide (CO2) has increased by 31% since 1750. The present CO2 concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The current rate of increase is unprecedented during at least the past 20,000 years.
	About three-quarters of the anthropogenic emissions of CO2 to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is predominantly due to land-use change, especially deforestation.
	Currently the ocean and the land together are taking up about half of the anthropogenic CO2 emissions. On land, the uptake of anthropogenic CO2 very likely exceeded the release of CO2 by deforestation during the 1990s.


	The atmospheric concentration of nitrous oxide (N2O) has increased by 46 ppb (17%) since 1750 and continues to increase. The present N2O concentration has not been exceeded during at least the past thousand years.
	About a third of current N2O emissions are anthropogenic (e.g., agricultural soils, cattle feed lots and chemical industry).


	The atmospheric concentration of methane (CH4) has increased by 151% (1060 ppb9) since 1750 and continues to increase.
	The present CH4 concentration has not been exceeded during the past 420,000 years.
	Slightly more than half of current CH4 emissions are anthropogenic (e.g., use of fossil fuels, cattle, rice agriculture and landfills).


	Since 1995, the atmospheric concentrations of many of those halocarbon gases that are both ozone-depleting and greenhouse gases (e.g., CFCl3 and CF2Cl2), are either increasing more slowly or decreasing, both in response to reduced emissions under the regulations of the Montreal Protocol and its Amendments.
	Their substitute compounds (e.g., CHF2Cl and CF3CH2F) and some other synthetic compounds (e.g., perfluorocarbons (PFCs) and sulphur hexafluoride (SF6)) are also greenhouse gases, and their concentrations are currently increasing.


	WMO


	Summary for Policymakers (SPM)


	The observed depletion of the stratospheric ozone (O3) layer from 1979 to 2000 is estimated to have caused a negative radiative forcing (–0.15 Wm^-²).
	The total amount of O3 in the troposphere is estimated to have increased by 36% since 1750, due primarily to anthropogenic emissions of several O3-forming gases. This corresponds to a positive radiative forcing of 0.35 Wm^-².


	The major sources of anthropogenic aerosols are fossil fuel and biomass burning. These sources are also linked to degradation of air quality and acid deposition.
	In general, the direct radiative forcing of aerosols is negative (except for black carbon fossil).
	There is much less confidence in the ability to quantify the total aerosol direct effect, and its evolution over time.
	Aerosols also vary considerably by region and respond quickly to changes in emissions.
	In addition to their direct radiative forcing, aerosols have an indirect radiative forcing through their effects on clouds.
	There is now more evidence for this indirect effect, which is negative, although of very uncertain magnitude.


	Since the late 1970s, satellite instruments have observed small oscillations due to the 11-year solar cycle. Mechanisms for the amplification of solar effects on climate have been proposed, but currently lack a rigorous theoretical or observational basis.
	Stratospheric aerosols from explosive volcanic eruptions lead to negative forcing, which lasts a few years. Several major eruptions occurred in the periods 1880 to 1920 and 1960 to 1991.
	The combined change in radiative forcing of the two major natural factors (solar variation and volcanic aerosols) is estimated to be negative for the past two, and possibly the past four, decades.


	Detection and attribution studies consistently find evidence for an anthropogenic signal in the climate record of the last 35 to 50 years.
	Simulations of the response to natural forcings alone (i.e., the response to variability in solar irradiance and volcanic eruptions) do not explain the warming in the second half of the 20th century.
	However, they indicate that natural forcings may have contributed to the observed warming in the first half of the 20th century.
	Most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.


	The globally averaged surface temperature is projected to increase by 1.4 to 5.8°C over the period 1990 to 2100.


	Based on global model simulations and for a wide range of scenarios, global average water vapor concentration and precipitation are projected to increase during the 21^st century.
	By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter.
	At low latitudes there are both regional increases and decreases over land areas.
	Larger year to year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected.


	Current projections show little change or a small increase in amplitude for El Niño events over the next 100 years.
	Even with little or no change in El Niño amplitude, global warming is likely to lead to greater extremes of drying and heavy rainfall and increase the risk of droughts and floods that occur with El Niño events in many different regions.


	It is likely that warming associated with increasing greenhouse gas concentrations will cause an increase of Asian summer monsoon precipitation variability.
	Changes in monsoon mean duration and strength depend on the details of emission scenario.


	Most models show weakening of the ocean thermohaline circulation which leads to a reduction of the heat transport into high latitudes of the Northern Hemisphere.
	The current projections using climate models do not exhibit a complete shut-down of the thermohaline circulation by 2100.
	Beyond 2100, the thermohaline circulation could completely, and possibly irreversibly, shut-down in either hemisphere if the change in radiative forcing is large enough and applied long enough. scenarios.


	Northern Hemisphere snow cover and sea-ice extent are projected to decrease further.
	Glaciers and ice caps are projected to continue their widespread retreat during the 21st century.
	The Antarctic ice sheet is likely to gain mass because of greater precipitation, while the Greenland ice sheet is likely to lose mass because the increase in runoff will exceed the precipitation increase.


	Global mean sea level is projected to rise by 0.09 to 0.88 metres between 1990 and 2100, for the full range of SRES scenarios.