When the earth grows too warm by way of increased emissions of the greenhouse gas carbon dioxide (as, it appears, has happened in the last few years) it seems the Earth has mechanisms to deal with it.
Increased carbon dioxide, the most common greenhouse gas, leads to a greening of the world - particularly in the northern latitudes. Plants require carbon dioxide and thrive in its presence. The secondary effect of a world "greening" is ultimately, a lowering of global temperature. Greater plant life absorbs more of the greenhouse gas carbon dioxide and releases oxygen.
This mechanism has limits. If the Earth's temperature rose too high or too quickly to cause the beneficial "greening" effect, you might see a scorched planet instead of a greener planet. The loss of rainforests is a significant blow to the earth's ability to handle carbon dioxide. And carbon dioxide isn't the only greenhouse gas we've been emitting.
Fortunately the earth has another mechanism for dealing with heat. Increased heat increases evaporation, which increases cloud cover. Clouds reflect more of the sun's heat and provide rain, which is of further benefit to the carbon-dioxide-absorbing plant life. Evidence that this mechanism is working came with the release last month of a study on earthshine. "Earthshine" (here's a picture) is the ghostly outline of light reflected from the Earth to the Moon and back again.
The new study, published in Science, was conducted by Enric Palle and colleagues at the Big Bear Solar Observatory in California and the California Institute of Technology in Pasadena. They report that the brightness of earthshine decreased steadily during the period 1984 to 2000, although that trend appears to have reversed since.
The decline suggests fewer clouds, which reflect sunlight, and therefore that more sunlight has been making it into the lower atmosphere (troposphere). That change is "consistent with the large tropospheric warming that has occurred over the most recent decades", they write.
Beside offering another explanation for increased temperatures other than greenhouse gases, the best part of this news is that Earthshine has increased since 2000. This suggests that cloud cover is returning and that we are in for cooler, rainier weather.
We know more about how a warm Earth cools than how a cold Earth heats up. In his book A Short History of Nearly Everything Bill Bryson said (page 429):
If Earth did freeze over, then there is the very difficult question of how it ever got warm again. An icy planet should reflect so much heat that it would stay frozen forever.
Bryson suggests that we've been saved during previous ice ages by volcanoes pumping out heat and greenhouse gases.
UPDATE: This afternoon Reuters reports:
In an article in the science journal Nature, Norwegian researchers said they had found traces of thousands of hydrothermal vents in lava off Norway that could have been the source of a rise in greenhouse gases 55 million years ago.
Until now, scientists have been at a loss to explain the trigger for a 5-10 Celsius (10-20F) global warming over about 10,000 years in the Eocene -- a blink in geological time.
"We think that magma heated sediments containing organic material and led to an explosive release of gases," said Henrik Svensen, a researcher at the University of Oslo and main author of the article.
The scientists said the annual rate of modern human emissions of greenhouse gases to the atmosphere in the 1990s -- from fossil fuels burned in cars, factories and power plants -- was 35-360 times as fast as the pace of the Eocene gas buildup…
"We can cause the same amount of global warming ourselves in a few hundred years at current rates," Svensen said.
I just can't work up too much panic over this. We can't say with any degree of certainty what fuels we will use 25 years from now, much less what our fossil fuel consumption patterns will be over the next "few hundred years."
…hundreds of instruments around the world recorded a drop in sunshine reaching the surface of Earth, as much as 10 percent from the late 1950's to the early 90's, or 2 percent to 3 percent a decade… Dr. James E. Hansen, director of the NASA Goddard Institute for Space Studies in Manhattan, said that scientists had long known that pollution particles reflected some sunlight, but that they were now realizing the magnitude of the effect… Satellite measurements show that the sun remains as bright as ever, but that less and less sunlight has been making it through the atmosphere to the ground. Pollution dims sunlight in two ways, scientists theorize. Some light bounces off soot particles in the air and goes back into outer space. The pollution also causes more water droplets to condense out of air, leading to thicker, darker clouds, which also block more light. Via KurzweilAIOf course increased evaporation also leads to thicker darker clouds. Global Warming is a complicated issue. We simply don't know enough about the effect of emissions, our planet's ability to cope, or even how technology will allow us to clean up the environment in the near future. We do know that the world's population requires modern technology to survive. And I suspect that increased technology will allow us to decrease emissions. In the meantime we should tread lightly.
Randall Parker reports that carbon dioxide may not be the actual culprit in global warming (if in fact there is a culprit at all):
While this may not be immediately obvious this report seems like good news. Why? Because it is a lot cheaper to reduce soot emissions than to reduce carbon dioxide emissions. If a substantial source of warming can be cancelled out cheaply then that buys time (assuming it really is necessary to intervene in the first place) to develop technologies that will allow carbon dioxide emissions to be reduced at much lower cost.
So let me see if I've got this straight. The world may be getting warmer, but then again, it may be getting colder. We aren't sure. If it is getting warmer, this maybe be due primarily to carbon dioxide emissions. Or it may be due to soot. Or (let's face it) it may be due to something else.
Then again, it may not be so much that the world is getting warmer or colder, maybe it's getting darker.
Whatever the heck is going on, it's obvious that we should start passing and enforcing some really restrictive laws pronto.
One of the reasons I love reading FuturePundit so much is that Randall Parker frequently introduces startling ideas that either completely refute conventional wisdom or that depart from it in totally unepected ways. First it was global cooling, now it's global dimming.
How anyone can be glib or confident in trying to explain the impact humanity has on climate is beyond me. As Randall eloquently says:
The scale of human activity has gotten so large that we inevitably change the climate to some extent. We do not know yet just how much we are changing the climate because we do not know what the climate would be like in our absence. Since the human population is growing and parts of the world are rapidly industrializing human influence on the climate looks set to grow even further. But since there are so many human activities that cause climate effects and since some of those effects cancel each other out (at least to some extent) any effort to reduce only a single pollutant or to reduce the impact of only a single method of modifying our environment will have the effect of strengthening the impact of other things that we do.
Indeed and indeed.