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Global warming?

I think I have officially two posts on this blog about global warming and I don’t feel like talking about religion or politics, so I figured this was useful to discuss.

One of the better points I see climate change deniers as having is the claim that our climate goes through warming and cooling cycles, so today’s global warming is completely natural and there’s nothing we can do about it. At least, it’s an interesting claim to me.

Supporting that argument is the claim that we produce little CO2 in comparison to natural sources. I’ve also heard in several places that volcanoes produce far more greenhouse gases than human beings do. That seems to be clearly wrong, according to the USGS, who say humans win 150 times over. However, the kernel of truth seems to be that humans produce little compared to the overall amount of CO2 produced by nature. But not so fast:

The Earth has a natural CO2 cycle that moves massive amounts of CO2 into and out of the atmosphere. The oceans and land vegetation release and absorb over 200 billion metric tons of carbon into and out of the atmosphere each year. When the cycle is balanced, atmospheric levels of CO2 remain relatively stable. Human activities are now adding about 7 billion metric tons of carbon into the atmosphere every year,which is only about 3–4% of the amount exchanged naturally. But that’s enough to knock the system out of balance, surpassing nature’s ability to take our CO2 emissions out of the atmosphere. The oceans and land vegetation are absorbing about half of our emissions; the other half remains airborne for 100 years or longer. This is what is causing the rapid buildup of CO2, a buildup that dwarfs natural fluctuations.

So, while we may not produce that much compared to natural causes, we put out enough to knock the balance out of whack. That still doesn’t answer the question I have. What exactly does that CO2 do in the atmosphere. According to one scientist,

“CO2 and climate are like two people handcuffed to each other,” he said. “Where one goes, the other must follow. Leadership may change, or they may march in step, but they are never far from each other. Our current CO2 levels appear to be far out of balance with climate when viewed through these results, reinforcing the idea that we have significant modern warming to go.”

The IPCC says it’s “virtually certain” that CO2 will be the main factor driving climate changes in this centure.

But don’t CO2 levels lag behind global temperature increases? Seems like a cause and effect problem. Well, maybe not:

The reason has to do with the fact that the warmings take about 5000 years to be complete. The lag is only 800 years. All that the lag shows is that CO2 did not cause the first 800 years of warming, out of the 5000 year trend. The other 4200 years of warming could in fact have been caused by CO2, as far as we can tell from this ice core data.

The 4200 years of warming make up about 5/6 of the total warming. So CO2 could have caused the last 5/6 of the warming, but could not have caused the first 1/6 of the warming.

It comes as no surprise that other factors besides CO2 affect climate. Changes in the amount of summer sunshine, due to changes in the Earth’s orbit around the sun that happen every 21,000 years, have long been known to affect the comings and goings of ice ages. Atlantic ocean circulation slowdowns are thought to warm Antarctica, also.

From studying all the available data (not just ice cores), the probable sequence of events at a termination goes something like this. Some (currently unknown) process causes Antarctica and the surrounding ocean to warm. This process also causes CO2 to start rising, about 800 years later. Then CO2 further warms the whole planet, because of its heat-trapping properties. This leads to even further CO2 release. So CO2 during ice ages should be thought of as a “feedback”, much like the feedback that results from putting a microphone too near to a loudspeaker.

Sadly, my ability to understand a lot of this ends at trying to evaluate how much CO2 affects global temperatures. The IPCC concluded that, outside of the early 20th century, natural causes can’t explain the warming, but greenhouse gases do. Specifically, it’s inconsistent with natural variability:

While these estimates vary substantially, on the annual to decadal time-scale they are similar, and in some cases larger, than obtained from observations. Estimates from models and observations are uncertain on the multi-decadal and longer time-scales required for detection. Nonetheless, conclusions on the detection of an anthropogenic signal are insensitive to the model used to estimate internal variability. Recent observed changes cannot be accounted for as pure internal variability even if the amplitude of simulated internal variations is increased by a factor of two or more. It is therefore unlikely (bordering on very unlikely) that natural internal variability alone can explain the changes in global climate over the 20th century

Which is where I started, I think.

Part of the problem with all this is that the science is amazingly complicated. When we have entities and groups on either side of the debate who aren’t entirely trustworthy (the fossil fuel industry and environmentalists, if you didn’t know) firing back and forth over on the subject it’s occasionally difficult to deal with. Eventually the distrust of the group that isn’t on our “side” takes over. That’s how I see it, at least. Still, I think a good faith effort to evaluate the scientific groups and their conclusions leads to clear cut support for global warming as a significant human caused problem.

Categories: Environment, Science
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