A Beginner's Guide to the IPCC Climate Change Reports

The first installment of the fifth Intergovernmental Panel on Climate Change report was published on Friday, Sept. 27, with more sections to be rolled out over the next year. The IPCC's broad conclusions have been remarkably consistent since the first report was published in 1990, although details have evolved.

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First Assessment Report, 1990

The Intergovernmental Panel on Climate Change was founded in 1988 to review the best research on climate change, evaluating both the risks and potential policy solutions. The First Assessment Report was released two years later.

Surface temperature: The panel predicted in one scenario that, if 1990's levels of emissions remained constant, the mean global temperature would increase by an average of about 0.3 degrees C per decade. This would be a more rapid increase than the globe had experienced during the past 10,000 years and would result in a rise of 3 degrees C by the end of the 21st century.

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Sea level rise: The report predicted that sea level would rise by 6 centimeters per decade over the next century under a "business as usual" scenario, in which greenhouse gas emissions were kept at 1990 levels. This rise would be caused mainly by thermal expansion of the ocean and the melting of some land ice. A 30- to 50-centimeter sea-level rise—which the panel said could happen by 2050 under the highest emission scenarios—would threaten low islands and coastal zones. A 1-meter rise by 2100 would have major ramifications, rendering some island countries uninhabitable, displacing tens of millions of people, threatening low-lying urban areas, flooding productive land, contaminating freshwater supplies, and seriously altering coastlines.

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IPCC notes: The projected changes in temperature and precipitation suggested to the IPCC that climatic zones could shift several hundred kilometers toward the poles over the next 50 years. Flora and fauna would lag behind, finding themselves in different environments, resulting in greater productivity for some species and population declines for others.

The IPCC also included a note saying that, because response processes were poorly understood, as the climate warmed there could be an overall increase in natural greenhouse gas abundances, and that climate change outcomes would likely be more severe than what the report was predicting.

Second Assessment Report, 1995

Surface temperature: The Second Assessment Report (SAR), published in 1995, included adjusted estimates for the global temperature increase. The report projected an increase in global mean surface temperature of about 1 to 3.5 degrees C by 2100, with the midline estimate being a 2-degree rise. This was about a third lower than the midline projection published in the 1990 report. However, the IPCC noted, even if greenhouse gas concentrations were stabilized by the end of the century, the temperature would continue to increase.

Sea level rise: The SAR was more conservative than earlier predictions in terms of sea level rise. Midrange models from the second report suggested that the sea level would rise by 50 cm by 2100, compared to 1995, less than the 1-meter threshold that had been predicted in 1990.

Popular Mechanics notes: The SAR raised the question of increased spending on air conditioning and refrigeration as global temperatures climbed. The Environmental Protection Agency currently estimates that if America's climate were to warm by 1.8 degrees Fahrenheit, the energy demand for cooling would increase by 5 to 20 percent. These changes in energy consumption would likely require costly improvements and adjustments to the energy infrastructure. According to the EPA, a national temperature increase of 6.3 to 9 degrees Fahrenheit would require additional electric generating capacity of about 10 to 20 percent by 2050, costing hundreds of billions of dollars.

Third Assessment Report, 2001

Surface temperature: The Third Assessment Report's (TAR) predictions regarding global temperatures were for bigger increases than those foreseen in the second report. Summarizing all scenarios addressed in the TAR, the global mean temperature was expected to rise by 1.4 to 5.8 degrees C by 2100. The panel said that this change was due mainly to lower projected sulfur dioxide emissions. (Particulate matter in the atmosphere can block incoming solar radiation, leading to a cooling effect.)

Sea level rise: The projected rise in sea level decreased from the 1995 report. Despite the higher projected temperature increase, the TAR predicted a rise of only 0.09 to 0.88 meters by 2100, primarily due to improvements in the climate models and a smaller predicted contribution from melting ice sheets and glaciers.

Emissions: As carbon-dioxide emissions increased, the TAR said, the oceans and terrestrial ecosystems—which act as sinks, absorbing greenhouse gases—would not be able to keep up. More carbon dioxide would be left in the atmosphere, and by 2100 the IPCC's carbon cycle models projected atmospheric concentrations of CO2 between 540 and 970 parts per million, 90 to 250 percent above the preindustrial level of the year 1750.

IPCC notes: The TAR included some bleak predictions about global warming's effect on human health. Regions where the transmission of malaria and dengue occur would spread past the 40 to 50 percent of the world they already affect. More frequent heat waves, along with increased humidity and urban air pollution would cause a spike in heat-related deaths and illnesses. More frequent flooding would result in additional health problems. These negative effects would be most severe in poorer countries.

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Popular Mechanics notes. According to the World Health Organization, malaria death rates have actually plummeted since 2000, decreasing by more than 25 percent globally due to improved prevention measures. Dengue fever, however, has become more prevalent, with 2.5 billion people currently living in regions at risk of infection.

Fourth Assessment Report, 2007

Surface temperature: The projections for the rise in global temperature rose again in the Fourth Assessment Report (AR4). The range of temperature increase by end of the 21st century was 1.1 to 6.4 degrees C across all scenarios. Should the temperature increase more than 2.5 degrees C beyond the mean 1999 temperature of 15.6 degrees C, the IPCC said it had "medium confidence" that 20 to 30 percent of known animal and plant species would be at increased risk of extinction. If the global average temperature increase exceeded 3.5 degrees C, models suggested that there would be extinctions of 40 to 70 percent of known species.

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Sea level rise: The IPCC's panels continued to home in on a more accurate picture of projected sea level rise. The fourth report predicted a 0.18- to 0.59-meter increase across all scenarios by 2099, compared with the TAR predictions of 0.09 to 0.88 meters. In 2007, the IPCC didn't incorporate the effect of glacial dynamics, pending more research, so the estimate of sea-level rise was considered conservative. The report also estimated with "high confidence" that by mid-century, water availability would increase at high latitudes and decrease at mid latitudes and in the tropics.

In some projections, Arctic late-summer sea ice disappeared almost entirely by the latter part of the 21st century. If global average warming were to continue for millennia at temperatures greater than 1.9 to 4.6 degrees C above preindustrial levels, the Greenland ice sheet would disappear almost entirely. This alone would eventually cause the sea level to rise by about 7 meters.

Emissions: Global greenhouse gas emissions were likely to spike, the report predicted, rising 25 to 90 percent between 2000 and 2030.

IPCC notes: The report predicted that world's oceans would become more acidic, with a reduction in ocean pH levels between 0.14 and 0.35 units during the 21st century. This was expected to have the greatest negative impact on shell-forming organisms, such as coral reefs, and their dependent species.

The report also predicted that extreme weather events, such as droughts, floods, and cyclones would occur more frequently and with greater intensity, and that these could spur increased incidences of wildfires and salinization of irrigation water and other freshwater systems.

Fifth Assessment Report, 2013

Surface temperature: The release of the IPCC's Fifth Assessment Report (AR5) on Friday gives us the most up-to-date look at research into climate change. According to the report, the temperature at the Earth's surface has warmed more in each of the past three decades than in any other 10-year span since 1850. This is true even though surface temperature warming has slowed in the past decade. The IPCC's Thomas Stocker said in a press release that global temperature increase by the end of the 21st century will likely exceed 2 degrees C, with the range of estimates running anywhere from 0.3 to 4.8 degrees C based on 2005 numbers. The IPCC reports with very high confidence that the Arctic region will continue to warm faster than the globe overall.

Sea level rise: The current IPCC assessment predicts that sea-level rise will accelerate. The most modest projections in the AR5 have the sea level rising by 0.26 to 0.55 meters by 2100 above 1986 levels, with an estimated rise of 0.52 to 0.98 meters in the more extreme scenario. The IPCC predicts with "medium confidence" that, by the end of the 21st century, the globe's total volume of glaciers, excluding the ice caps, will decrease anywhere from 15 to 85 percent.

Emissions: The 15 climate models the IPCC used for the AR5 projected widely divergent cumulative carbon-dioxide emissions from 2012 to 2100, ranging from 140 to 1910 gigatonnes.

IPCC notes: Even if we were to halt our CO2 emissions immediately, this IPCC report states, many of the effects of continued carbon-dioxide emissions would be irreversible for hundreds or thousands of years. Depending on the scenario, it says, about 15 to 40 percent of emitted CO2 would remain in the atmosphere longer than 1000 years.

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