The IPCC synthesis report

Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.³

I knew about the pattern in global temperatures – roughly a 2°F increase since 1850, most of it in the last half century – but the rise in global sea level is much greater than I thought – roughly 8 inches since 1870. Eight inches may not sound like much, but ask someone who has coastal property on a nice, sandy beach how much difference eight inches could make.

Most of the observed increase in globally-averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG concentrations. It is likely there has been significant anthropogenic warming over the past 50 years averaged over each continent (except Antarctica)

It takes a lot of heavy duty math, physics, and computer modeling to produce the climate models used to produce the figure to the left, but it doesn't take any to see that the observed average temperature (the thin black line) falls nicely inside the model projections that include anthropogenic forcings. The figure not only shows why the IPCC concluded that human activities are responsible for most global warming in the last half-century, it also shows that the models they use to project climate change into the next century work very well.

Continued GHG [greenhouse gas] emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.

The pink line in this figure corresponds to holding greenhouse gas levels in the atmosphere at year 2000 levels. That obviously won't happen. The IPCC included it to provide a baseline for comparison. None of the scenarios illustrated here include climate policies. They illustrate what is likely to happen if we do nothing. The lowest end of the plausible range among all scenarios is for global average temperature to be about 2.5°F higher than its 1980-1999 average. The highest end is about 11.5°F. The range of best estimates for the six scenarios is 3.5-7.0°F.

Of course, the globe will not warm uniformly. In Arctic regions, the average temperature under a mid-range emissions scenario (A1B) could be as much as 14-15°F higher than the 1980-1999 average. In most of the rest of the world the increase is projected to be only about half as much. The uneven distribution of climate impacts across regions is one of the aspects of climate change that is too little appreciated by most people (and too little emphasized by most media outlets). Not only will the global average temperature rise significantly as a result of climate change, but the frequency and magnitude of extreme events will increase. For example, the IPCC concludes that

• it is very likely we'll see increases in the frequency of heat waves and heavy precipitation,
• it is likely we'll see more tropical cyclones, i.e., hurricanes, and
• there is high confidence that many arid and semi-arid areas will have fewer water resources than they do now.

I'll quote a few more points verbatim without comment:

• Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.
• There is high confidence that neither adaptation nor mitigation alone can avoid all climate change impacts; however, they can complement each other and together can significantly reduce the risks of climate change.
• Delayed emission reductions significantly constrain the opportunities to achieve lower stabilisation levels and increase the risk of more severe climate change impacts.
• There is high agreement and much evidence that all stabilisation levels assessed can be achieved by deployment of a portfolio of technologies that are either currently available or expected to be commercialised in coming decades, assuming appropriate and effective incentives are in place for their development, acquisition, deployment and diffusion and addressing related barriers.

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¹11:00am in Valencia.

²Since downloads from the IPCC site seem to be pretty small, a copy of the “Summary for Policymakers” is also available here.

³All of the quotes are direct from the “Summary for Policymakers.”