1) Average Surface Temperature
There is robust evidence for a 0.8 degree increase in the global surface temperature since preindustrial times, confirmed by four independent analyses. The ten hottest years on record all come after 1997. Temperature measurement sites cover the globe sufficiently finely (angular separation of about one degree) to allow accurate averages. Two objections have been raised to these results. First, it is claimed that they may be skewed by the presence of some measurement sites in urban heat islands. This is a legitimate concern, but it is adequately countered in the averaging process, which rejects measurements in urban areas if they disagree with neighboring rural measurements. Besides, if one rejects urban sites altogether and uses only uncontaminated rural sites, the warming trend is stronger. Another objection is that satellite data failed to confirm a warming trend in the mid-atmosphere over the last couple of decades. This was indeed a notable discrepancy, but it is now generally accepted that it came from imperfections in the statistical analysis of the satellite data. A more recent analysis incorporating both satellite and balloon data confirms mid-altitude warming. (The stratosphere, on the other hand, has cooled significantly over the same period that the troposphere has warmed. As we will see, this is a robust prediction of greenhouse effect models and is inconsistent with warming by an increase in solar radiation.)
Average surface temperature of the Earth (averages from direct measurements)
In the data above, we see significant short-term variations on top of a clear overall trend. It would thus be reckless to read too much into the apparent stall in heating over the past decade (which is still, you’ll recall, the warmest since direct measurements began). In fact, given that solar irradiance is going through a trough, the real notable fact is that the world isn’t cooling. (The difference in solar forcing between peak and trough is roughly the same as a little over a decade of CO2 dumping at expected rates, according to models whose veracity we will consider later.)
The most notable thing about the above curve is that it does not track the surface temperature.
2) Ocean Heating
Ultimately, ocean temperature is a far better measure of global warming than surface air temperature, since the ocean stores far more heat than the atmosphere. The heat content of the ocean shows an unmistakable warming trend:
Ocean heat energy
This has led to an accompanying rise in sea levels
Glaciers and ice caps are melting in many places, although regional variations are large.
The average concentration of carbon dioxide (usually thought to be the ultimate driver behind changes in the atmosphere’s infrared opacity, i.e. increases in other greenhouse gases like water vapor are thought to be caused by heating from CO2 increase) has increased from 260-280ppm before the Industrial revolution (according to ice core measurements staying within this range for millennia) to 391 today, and rapidly increasing. Humans emit more than enough to account for this (indicating that carbon trapping by oceans, plants, or whatever has slightly accelerated, so that we don’t bear the full brunt of whatever the effects would otherwise be of our emissions).
Past temperatures can be estimated indirectly from tree rings, ice core bubbles, and other markers. However, these measurements seem to me much more uncertain than the above results. In order to avoid diversion into unnecessary controversy, I will not discuss or refer to them. I will also not discuss the claim, plausible but not definitely established, that global warming is or will make extreme weather phenomena more common (e.g. stronger hurricanes).
The 1970-2000 warming seems undeniable. Even if the methodology of one of these measurements were questionable, it is quite implausible that all of them are wrong while remaining consistent with each other. If, for example. someone thinks the surface land temperature measures are flawed or manipulated, how does he explain the ocean temperature, sea level, glacier, permafrost, upper atmosphere, etc. results, all of which corroborate the basic warming theory?
The important questions, to be addressed in successive posts, are
What’s causing the warming?
To what degree will warming continue in the future?
How bad will the effects be?
If we’re causing it, are the effects bad enough to offset the cost of stopping it?
Filed under: Climate