The big temperature picture. Graph and insight from Dr Syun Akasofu (2009 International Conference on Climate Change, New York, March 2009).]
The pattern suggests that the world has entered a period of slight cooling until about 2030.
The global temperature has been rising at a steady trend rate of 0.5°C per century since the end of the little ice age in the 1700s (when the Thames River would freeze over every winter; the last time it froze over was 1804). On top of the trend are oscillations that last about thirty years in each direction:
1882 – 1910 Cooling
1910 – 1944 Warming
1944 – 1975 Cooling
1975 – 2001 Warming
In 2009 we are where the green arrow points, with temperature leveling off. The pattern suggests that the world has entered a period of slight cooling until about 2030.
There was a cooling scare in the early 1970s at the end of the last cooling phase. The current global warming alarm is based on the last warming oscillation, from 1975 to 2001. The IPCC predictions simply extrapolated the last warming as if it would last forever, a textbook case of alarmism. However the last warming period ended after the usual thirty years or so, and the global temperature is now definitely tracking below the IPCC predictions.
The IPCC blames human emissions of carbon dioxide for the last warming. But by general consensus human emissions of carbon dioxide have only been large enough to be significant since 1940—yet the warming trend was in place for well over a century before that. And there was a cooling period from 1940 to 1975, despite human emissions of carbon dioxide. And there has been no warming since 2001, despite record human emissions of carbon dioxide.
There is no actual evidence that carbon dioxide emissions are causing global warming. Note that computer models are just concatenations of calculations you could do on a hand-held calculator, so they are theoretical and cannot be part of any evidence. Although the models contain some well-established science, they also contain a myriad of implicit and explicit assumptions, guesses, and gross approximations—mistakes in any of which can invalidate the model outputs.
Furthermore, the missing hotspot in the atmospheric warming pattern observed during the last warming period proves that (1) the IPCC climate theory is fundamentally broken, and (2) to the extent that their theory correctly predicts the warming signature of increased carbon dioxide, we know that carbon dioxide definitely did not cause the recent warming (see here for my full explanation of the missing hotspot). The alarmists keep very quiet about the missing hotspot.
No one knows for sure what caused the little ice age or for how many more centuries the slow warming trend will continue. It has been warmer than the present for much of the ten thousand years since the last big ice age: it was a little warmer for a few centuries in the medieval warm period around 1100 (when Greenland was settled for grazing) and also during the Roman-Climate Optimum at the time of the Roman Empire (when grapes grew in Scotland), and at least 1°C warmer for much of the Holocene Climate Optimum (four to eight thousand years ago).
Measuring the global temperature is only reliably done by satellites, which circle the world 24/7 measuring the temperature over large swathes of land and ocean. But satellite temperature records only go back to 1979. Before that, the further back you go the more unreliable the temperature record gets. We have decent land thermometer records back to 1880, and some thermometer records back to the middle of the 1700s. Prior to that we rely on temperature proxies, such as ice cores, tree rings, ocean sediments, or snow lines.