The Berkeley Earth Surface Temperature project concludes: carbon dioxide concentration and volcanic activity suffice to explain most of the changes in earth’s surface temperature from 1751 to 2011. Carbon dioxide increase explains most of the warming; volcanic outbursts explain most of the bits of sudden cooling. The fit is not improved by the addition of a term for changes in the behavior of the Sun!
For details, see:
• Robert Rohde, Richard A. Muller, Robert Jacobsen, Elizabeth Muller, Saul Perlmutter, Arthur Rosenfeld, Jonathan Wurtele, Donald Groom and Charlotte Wickham, A new estimate of the average earth surface land temperature spanning 1753 to 2011, Geoinformatics and Geostatics: an Overview 1 (2012).
The downward spikes are explained nicely by volcanic activity. For example, you can see the 1815 eruption of Tambora in Indonesia, which blanketed the atmosphere with ash. 1816 was called The Year Without a Summer: frost and snow were reported in June and July in both New England and Northern Europe! Average global temperatures dropped 0.4–0.7 °C, resulting in major food shortages across the Northern Hemisphere. Similarly, the dip in 1783-1785 seems to be to due to Grímsvötn in Iceland.
(Carbon dioxide goes up a tiny bit in volcanic eruptions, but that’s mostly irrelevant. It’s the ash and sulfur dioxide, forming sulfuric acid droplets that help block incoming sunlight, that really matter for volcanoes!)
It’s worth noting that they get their best fit if each doubling of carbon dioxide concentration causes a 3.1 ± 0.3°C increase in land temperature. This is consistent with the 2007 IPCC report’s estimate of a 3 ± 1.5°C warming for land plus oceans when carbon dioxide doubles. This quantity is called climate sensitivity, and determining it is very important.
They also get their best fit if each extra 100 gigatonnes of atmospheric sulfates (from volcanoes) cause 1.5 ± 0.5°C of cooling.
They also look at the left-over temperature variations that are not explained by this simple model: 3.1°C of warming with each doubling of carbon dioxide, and 1.5°C of cooling for each extra 100 gigatonnes of atmospheric sulfates. Here’s what they get:
The left-over temperature variations, or ‘residuals’, are shown in black, with error bars in gray. On top is the annual data, on bottom you see a 10-year moving average. The red line is an index of the Atlantic Multidecadal Oscillation, a fluctuation in the sea surface temperature in the North Atlantic Ocean with a rough ‘period’ of 70 years.
Apparently the BEST team places more weight on the Atlantic Multidecadal Oscillation than most climate scientists. Most consider the El Niño Southern Oscillation to be more important in explaining global temperature variations! I haven’t seen why the BEST team prefers to focus attention on the Atlantic Multidecadal Oscillation. I’d like to see some more graphs…