Now, the IPCC is preparing to issue its 5th assessment later this year, and some of its drafts have already been circulating on the web. We can surely expect fireworks. Many still consider climate science to be a “massive hoax,” and they won’t take kindly to a synthesis of global science that provides more certainty regarding the trends in climate disruptions.
But before the new edition comes out, it might be good to know what the old one actually said. Of course, you don’t actually want to read it yourself. The thing consists of four volumes, covering basic science, vulnerabilities, mitigation, and a synthesis report. It’s so cumbersome that most commentators settle for reading only a Summary for Policymakers.
But it can be read, and we’ve done so. We thought that it might be a service to summarize – in layman’s terms – the Physical Science Basis Report. This tells us the state of basic climate science six years ago. A lot more is known today, but here’s a readable summary of what was known back then. We paraphrase most everything, but our account is an honest every-man’s rendering of largely impenetrable scientific language.
1. Greenhouse gases are increasing: Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities and now far exceed pre-industrial values.
- Highest recorded carbon levels: Carbon dioxide is the most important anthropogenic greenhouse gas. The global concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005, and now exceeds by far the natural range over the last 650,000 years. (Note: CO2 is now at 395.55 ppm.)
- Faster growth in carbon: The annual carbon dioxide concentration growth rate was larger during the last 10 years than it has been since the beginning of continuous direct atmospheric measurements.
- It comes from burning coal and oil: The primary source of the increased atmospheric concentration of carbon dioxide since the pre-industrial period results from fossil fuel use.
2. Warming of the climate system is unequivocal, as is now evident from observations of increases in global air and ocean temperatures, widespread melting of snow and ice, and rising global average sea levels.
- These are the hottest years: Eleven of the last twelve years (1995–2006) rank among the 12 warmest years on record (since 1850). (Note: The 6 years since then have all been hotter than every year before 1997, and include the #1, #5, and #6 hottest on record.)
- The pace of warming is accelerating: The linear warming trend over the last 50 years (0.13°C per decade) is nearly twice that for the last 100 years.
- The oceans are getting hotter: Observations since 1961 show that the average temperature of the global ocean has increased to depths of at least 3,000 meters and that the ocean has been absorbing more than 80% of the heat added to the climate system. Such warming causes seawater to expand, contributing to sea level rise.
- Glaciers are melting: Mountain glaciers and snow cover have declined on average in both hemispheres. Widespread decreases in glaciers and ice caps have contributed to sea level rise.
- Polar ice is melting: Losses from the ice sheets of Greenland and Antarctica have very likely contributed to sea level rise from 1993 to 2003.
- Seas are rising: Global average sea level rose faster in the 20th century than in the 19th century; faster yet in 1961-2003; and nearly double that rate during 1993-2003. Everything is accelerating.
3. Global warming is affecting the Polar Regions and the tropics most significantly.
- The Arctic is melting: Average Arctic temperatures increased at almost twice the global average rate in the past 100 years. Since 1978, annual average Arctic sea ice coverage has shrunk by 2.7% per decade, with larger decreases in summer of 7.4% per decade.
- The permafrost is thawing: Temperatures at the top of the permafrost layer have increased since the 1980s in the Arctic by 3°C. The maximum area covered by seasonally frozen ground has decreased by about 7% in the Northern Hemisphere since 1900.
- It’s dryer in Africa, and wetter in the Americas: Significantly increased precipitation has been observed in eastern parts of North and South America, northern Europe and northern and central Asia. Drying has been observed in the African Sahel, the Mediterranean, southern Africa and parts of southern Asia.
- Drought for the tropics: More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Increased drying linked with higher temperatures and decreased precipitation has contributed to increases in drought.
- More intense flooding storms: The frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmospheric water vapor.
- Stronger hurricanes: There is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic since 1970, correlated with increases of tropical sea surface temperatures.
4. The warmth of the last half century is unusual in at least the previous 1,300 years.
- This doesn’t happen regularly: Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1,300 years.
- When it does, oceans rise significantly: Looking way back, global average sea level in the last interglacial period (about 125,000 years ago) was likely 4 to 6 meters higher than during the 20th century. Ice core data indicate that average polar temperatures at that time were 3°C to 5°C higher than present.
5. We can no longer say “It’s not our fault.” Most of the observed increase in global average temperatures since the mid-20th century is very likely due to increases in human-caused greenhouse gases.
- Natural cycles have actually reduced warming: Increases in greenhouse gas concentrations alone would have caused more warming than observed because volcanic and human-caused aerosols have offset some warming that would otherwise have taken place.
- Nature hasn’t caused these events: It is extremely unlikely that global climate change of the past 50 years can be explained without external factors, and very likely that it is not due to known natural causes alone.
6. For the next two decades, a warming of about 0.2°C per decade is projected. Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected.
- With more greenhouse gases, the Earth must get hotter before reaching equilibrium: Warming is likely to be in the range 2°C to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C.
- Actual observed warming confirms the IPCC’s initial warnings: In the first report in 1990, projections were for global average temperature increases between about 0.15°C and 0.3°C per decade for 1990 to 2005. This can now be compared with observed values of about 0.2°C per decade.
7. Current trends spell accelerated warming: Continued greenhouse gas emissions at or above current rates will cause further warming and cause many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.
- It will be at least 1.8°C hotter this century: Best estimates for surface air warming in the 21st century is 1.8°C and the best estimate for the high scenario is 4.0°C.
- A hotter Earth will mean even more carbon emissions: Warming tends to reduce land and ocean uptake of atmospheric carbon dioxide, increasing the fraction of carbon emissions that remain in the atmosphere.
- Sea levels will rise further: Projections of global average sea level rise at the end of the 21st century are 0.3 to 0.6 meters, with most of the models projecting 0.2-.0.3 meters of sea level rise, excluding the impact of increases in polar ice flow. (Note: Most projections since this report point to much higher levels of sea level rise.)
- It could be much worse: Models used so far do not include uncertainties in climate-carbon cycle feedback nor do they include the full effects of changes in polar ice flow, because published research was lacking at the time.
- White, reflective snow and ice will be replaced by dark land and oceans: Snow cover is projected to decrease. Widespread increases in thaw depth are projected over most permafrost regions. Sea ice is projected to shrink in both the Arctic and Antarctic. In some projections, Arctic late-summer sea ice disappears almost entirely.
- More extreme weather ahead: It is very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent. It is likely that future tropical cyclones (typhoons and hurricanes) will become more intense.
- It will be dryer in the tropics, and wetter nearer the poles: Increases in the amount of precipitation are very likely in high latitudes, while decreases are likely in most subtropical land regions.
8. No quick fixes: Human-caused warming and sea level rise will continue for centuries due to the time scales associated with climate processes and feedbacks, even if greenhouse gas concentrations were to be stabilized.
- Big cuts in carbon are necessary just to stabilize: Based on current understanding of climate-carbon cycle feedback, studies suggest that to stabilize at 450 ppm carbon dioxide could require that cumulative emissions over the 21st century be reduced by 27% from 2006 levels.
- Sea levels will keep rising regardless of what we do: If greenhouse gases were to be stabilized at 2006 levels, thermal expansion alone would lead to 0.3 to 0.8 m of sea level rise by 2300 (relative to 1980–1999). Thermal expansion would continue for many centuries, due to the time required to transport heat into the deep ocean. Melting of Greenland Ice is projected to continue to contribute to sea level rise after 2100, and could result in a rise in sea levels of about 7 meters.
- This struggle will go on for centuries: Both past and future human-caused carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the time scales required for removal of this gas from the atmosphere.