Summary of the IPCC Report on the Physical Basis of Climate Change

Summary of IPCC Report

Paul Alois, February 2007

The Intergovernmental Panel on Climate Change (IPCC) is a United Nations sponsored task force that aggregates and analyzes the science behind human induced climate change. The IPCC does not conduct research or actively monitor data; instead its function is to collect original research being produced around the world and synthesize it.

The IPCC periodically releases reports on its findings, and in February 2007 it released a highly anticipated report titled “Climate Change 2007: The Physical Science Basis”. This report is the first of four volumes the IPCC will eventually publish on the overall picture of climate change. The report is the work of over 2,500 scientific experts, 800 contributing authors, and 450 leading authors who represent over 130 countries.

Since the IPCC’s previous report in 2001, the debate around climate change has intensified dramatically. Scientists, politicians, academicians, and even celebrities have come forth to support both sides of the argument. In this heated milieu the IPCC plays an important role. Its reports are generated by an enormous number of researchers from around the globe who, being relatively free of political pressure, are able to give an look at the science behind climate change. The following paper will summarize their most recent findings and elaborate on the highlights.

Drivers of Climate Change

The IPCC measures a number of factors affecting climate change and assesses them by their radiative forcing (RF), which is the extent to which a given factor impacts the balance between energy entering and exiting the planetary system. Each factor is assigned an RF number[1], with positive numbers denoting an increase in global temperatures and a negative number denoting a decrease in global temperatures. Each variable discussed below will have an RF number.

• Carbon dioxide emissions have an RF number of 1.6, making them the most significant contributor to climate change. The greatest source of carbon dioxide emissions is the burning of fossil fuels, although changes in land use also contribute. Since pre-industrial times carbon dioxide concentrations in the atmosphere have increase from 280 ppm3 to 370 ppm3.[2]
• Methane emissions have an RF number of 0.48, and the vast majority of methane emissions are generated by agriculture. Since pre-industrial times methane concentrations have increased from 715 ppb[3] to 1774 ppb.
• Ozone depleting chemicals[4] have an RF number of 0.35.
• Halocarbons have an RF number of 0.34.
• Nitrous oxide emissions have an RF number of 0.16.

All of the above variables contribute to a warming of the global climate. Their combined effect along with the addition of several small factors totals an RF number of 3.14. The following variable all contribute to a cooling of global temperatures.

• The emission of aerosols[5] has an RF number of -1.2. The direct reduction of radiative forcing is responsible for -0.5, while the indirect creation of cloud cover is responsible for -0.7 RF.
• Human induced changes in land cover have an RF number of -0.2.

These two factors have a combined RF number of -1.4, and when that is added to the RF number associated with factors causing global warming the final RF number is 1.87. However, due to uncertainties in the data the IPCC reports that an increase of 1.6 RF is a more likely number. While the IPCC does not specify an exact correlation between RF and temperature, Dr. James E. Hansen of the NASA Goddard Institute states that an increase of 1 RF equals an increase in the global temperature of 0.75° C.[6] The report also concludes that there is 90% chance that the “globally averaged net effect of human activities since 1750 is one of warming.”

Observations of Climate Change

With the advent of sophisticated climate monitoring technology in the last fifty years, climatologists have been able to very accurately measure specific aspects of the global climate. The IPCC reports that “warming of the climate system is unequivocal”, and cited increases in global average air and water temperatures, snow and ice melting, and rising sea levels.

• 11 of the last 12 years have been the warmest years on the planet since 1850.
• Between 1906 and 2005 the global temperature increased 0.74° C.
• Per decade warming rates in the last 50 years are twice as high as rates in the last 100 years.
• The average temperature of the ocean is decreasing, and it appears that the ocean is absorbing 80% of the increases in the global system.
• Mountain glaciers and snow cover have declined in both hemispheres.
• There is an 80% chance that sea levels rose from the 19th to 20th centuries.
• Between 1961 and 2003 sea levels rose 1.8 mm per year.
• Between 1993 and 2003 sea levels rose 3.1 mm per year, although it is unclear whether this was caused by climate change or by decadal fluctuations.
• Artic temperatures have increased at twice the global rate for the last 100 years.
• Since 1978 artic ice levels have decreased by 2.7% per decade; decreases in the summer have been 7.4% per decade.
• Antarctic ice levels have not shown significant longitudinal change.
• There is insufficient evidence to suggest that the circulation of the global ocean has been affected.

Attributing Climate Change

The IPCC reports that there is a 90% chance that the observed temperature increases of the last fifty years are the result of greenhouse gas emissions. Furthermore, human activities have begun affecting specific aspects of the climate such as heat waves, wind patterns, and continental temperatures.

• There is a greater than 66% chance that global warming would be much more significant where it not for the cooling effects of aerosols.
• There is a 5% chance that climate change can be explain without external forcing, and a 10% it can be attributed to natural forces alone.
• There is a greater than 66% chance that human forces have changed wind patterns.
• There is a greater than 50% chance human forces have increased the risk of heat waves.

Projections of Future Changes

The IPCC developed six scenarios using differing projections of global economic development, per capita consumption, implementation of clean energy sources, and fertility rates. It also makes projections about specific aspects of the planetary system. Models do not take into account feedback cycles, changes in ice flow, or climate change initiatives like the Kyoto Treaty.

• The most conservative scenario estimates that the global average temperature will increase by 1.1° to 2.9° C in the next century. This scenario assumes that the global population will begin falling mid-century, the onset of a service and information economy, and the widespread use of clean technology.
• The most extreme scenario estimates an increase in the global average temperature of 2.4° to 6.4° C in the next century. This scenario assumes rapid economic growth, a population that peaks mid-century, and a heavy reliance on fossil fuel generated energy.
• In the most conservative scenario, sea levels rise 1.8 to 3.8 meters by 2100.
• In the most extreme scenario, sea levels rise 0.26 to 0.59 meters by 2100.
• The carbon dioxide and methane absorbed by the oceans turns into carbonic acid, altering the ocean’s pH balance. The ocean’s pH level has already been lowered by .1 units, and in the next century could be lowered by 0.14 to 0.35.
• Warming will be greatest over land and high northern latitudes.
• Snow cover is expected to decrease.
• Sea ice is expected to decrease in both poles.
• There is a 90% chance that heat waves and intense precipitation events will become more frequent.
• There is a 90% that high latitudes will have more precipitation,
• There is a greater than 66% chance that sub-tropical latitudes will have less precipitation.
• There is a 90% chance that ocean circulation in the Atlantic will slow down, although there is only a 10% chance that is will undergo abrupt change.
• Assuming that radiative forces are balanced by 2100, in the following century the global temperature will continue to rise by 0.5° and the sea levels will rise by 0.3 to 0.8 meters.

Conclusion

The broad strokes of the report show that the IPCC believes there is a 90% chance that the Earth is getting warmer, a 90% chance that humans have caused it, and it is a virtual certainty that the warming will continue into the next century. This report is only the first of four installments of a larger publication; the remaining installments will discuss the socio-economic and environmental impacts of climate change, and examine mitigation strategies.

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