Ruddiman sees the signals of human interference in the pattern of methane and CO2 in the atmosphere from old antarctic ice cores. About 8000 years ago the CO2 pattern diverged from previous inter-glacial patterns, starting to rise after 2500 years of falling; this seems to coincide with humans starting to burn down forests for agriculture. Then about 5000 years ago the methane pattern also abruptly shifted, which Ruddiman attributes mainly to the flooding of Asian rice fields, still a major source of atmospheric methane.
Not every climate scientist agrees with Ruddiman – strangely enough, some global warming deniers have come out in support of Ruddiman – even though he’s talking about global warming effects due to human-caused greenhouse gas increases that match very closely climate models, the very thing these guys usually like to deny!
RealClimate has more on the ironies here. The main point, of course, is that the human-caused changes in greenhouse gases of the past 8000 years, if that’s really what they were, have already been doubled by the fossil fuel burning of recent decades. Plotting CO2 concentrations, for example, on the same graph as Ruddiman’s for the past 8000 years, you get a huge vertical spike for the last 100 years. It’s that near-vertical spike in CO2 that we need to worry about, not the gradual millenial changes. I’m sure we’ll figure out how to engineer the world over the next thousand years or so to keep it at whatever glaciation level we like – as long as we survive the next hundred, that is!
Perhaps Ruddiman can tell us why global average temperatures are yet to exceed their 1998 levels despite carbon dioxide levels rising since that year by about 11 ppm, equivalent to about 14% of the total increase since 1900.
If carbon dioxide is causing the warming, then where is it? And don’t try claiming that it takes 20-30 years to appear – carbon dioxide increased almost as much in any 6 year period in the 1970s and 1980s.
You might have seen NASA’s James Hansen trying to explain the 1998 temperature. He said, after years of claiming carbon dioxide was the cause, that the warming in 1998 was made worse by a severe El Nino that year and that a weak(!) El Nino pushed up temperatures in 2002 and 2003.
Get it?
NASA says that even weak natural events have an influence on temperature.
Put the two comments together and we have to ask what makes anyone so sure that humans anything at all to do with climate change?
cheers
My understanding of how global warming works is that extra heat trapped in the atmosphere adds energy to the world’s weather systems. This will not necessarily show up as drastic temperature increases the world over, but rather as more intense and unpredictable weather patterns and storms.
Temperatures will bump up or down randomly a bit from year to year, just as with any natural system. The total averaged increase expected so far is less than 1 degree Celsius, and random fluctuations year to year are certainly on that order. The real change is the year-after-year changes in the average, and of course the regional changes that may have larger amplitudes (for example much greater warming in the Arctic).
Climate is the long-time average of Earth’s atmosphere and ocean behavior; weather is short-time behavior. Weather is obviously much less predictable; averages at the one-year level aren’t quite at the predictability level of climate, but over a decade they seem to be.
Lots more on this and related items at RealClimate.