The Earth has gone through big climatic changes in its multi-billion year history. Global warming deniers point to these and say, "See, nature continually warms and cools the Earth, so there's no need to worry about what people are doing to the climate."
That's ignorance talking, as I learned in reading more of James Hansen's fascinating book, "Storms of My Grandchildren: The Truth About the Coming Climate Catastrophe and Our Last Chance to Save Humanity."
If that subtitle sounds dramatic to you, be assured that it isn't.
About four years ago TIME magazine, a genuinely fair and balanced news source, had a cover story about global warming titled "Be Worried. Be Very Worried." I wrote about it in "Global warming is real. Debate over."
Unfortunately, this guy graphically shows why global warming can't be a conspiracy, but denial can.
People are gullible and prone to believing bullshit rather than facts. A few errors in the IPCC (Intergovernmental Panel on Climate Change) are utterly insignificant in light of the fact, according to the New York Times, that the 2007 IPCC report cites 10,000 scientific papers and is more than 3,000 pages long.
Few people are going to take the time to become familiar with that vast body of research. That's why popularizers like James Hansen (director of the NASA Goddard Institute for Space Studies) are so important.
They show us the most important information we need to know about how natural global climate change has operated in the past, which gives us great clues about how man made global warming will change Earth in the future.
Hansen introduces the figure below with:
Even the most hardened antiscience zealot, once he understands figure 18, will have to admit that it is one of the most beautiful curves on the planet (I'm referring to scientific curves). It contains an enormous amount of interesting information about Earth's history. There are remarkable stories in both the broad sweep of climate over the 65 million years and in the rapid climate fluctuations.
FIGURE 18. Deep ocean temperature during the Cenozoic era. (See text. Original data from Zachos et al., "Trends, Rhythms, and Aberation in Global Climate 65 Ma to Present. "See sources.)
The "PETM" on the figure stands for Paleocene-Eocene thermal maximum. Fifty million years ago, Hansen says, Alaska had tropical-like vegetation and crocodiles. But during recent ice ages, which occurred on the right side of the long-term cooling trend shown in blue, ice sheets reached as far south as Kansas.
So what caused the global warming that peaked 50 million years ago?
Hansen describes how the sun's brightness has increased .4% over the past 65 million years, while Earth has actually cooled, so the sun can't be the cause of the climate changes shown in the figure.
He also says that changes in the location of continents (more about this below) has just been a minor influence on global climate.
However, carbon dioxide in the atmosphere has varied from 170 ppm (parts per million) in recent ice ages to 1,000 to 2,000 ppm in the early Cenozoic -- when those crocodiles were cruising around Alaska.
So carbon dioxide changes in the Cenozoic caused a forcing of about 12 watts [per square meter, averaged over the planet] -- at least ten times greater than the climate forcing due to either the sun or Earth's surface. It follows that changing carbon dioxide is the immediate cause of the large climate swings over the last 65 million years.
Since people weren't around, burning fossil fuels, during almost all of that period, where did the carbon dioxide come from that caused the massive global warming 50 million years ago?
Hansen points to this figure as the answer:
FIGURE 19. Continental locations 65 million years ago and today. The Cretaceous era ended and the Cenozoic began 65 million years ago. (Data from Hansen et al., "Target Atmospheric CO2," (see sources) based on original data from Ron Blakey at Northern Arizona University.)
As the continents move, Hansen says, they can ride over ocean crust -- which has massive amounts of calcium carbonate and organic sediments. That heat and pressure produces carbon dioxide and methane.
Note how modern-day India cruised through the Indian Ocean until it reached Asia. This was an area where major rivers had deposited carbon sediments for a long time.
Undoubtedly, the carbon-rich sediments on that ocean floor were subducted beneath the Indian continental plate. Then, 50 million years ago, India crashed into Asia, with the Indian plate sliding under the Asian plate... With India's sojourn across the carbon-rich ocean completed, the carbon dioxide emissions declined and the planet began a long-term cooling trend.
If you want a much more detailed explanation, here's the paper Hansen cites in his book:
Now, seemingly it's reasonable to think, "OK. So what? India isn't going to repeat its crash-into-Asia thing. Why should we care about this today?"
A couple of reasons. Great reasons.
First, Hansen says that the methane hydrate reservoir that the ancient carbon dioxide emissions came from is fully charged again. It's possible that human-caused global warming could start to melt the methane on continental shelves, leading to runaway further global warming.
Which wouldn't be good, given that when ice sheets melted, sea level 13,000 to 14,000 years ago rose 10 to 17 feet per century for several centuries. It was only when sea level stabilized that human civilization really took off.
Second, the long-term paleoclimate data shows us what happens when carbon dioxide levels rise or fall to certain levels. This is what really did happen on Earth, not what computer models predict might happen.
Read this, and weep. For your grandchildren and future generations, if you're as old as Hansen (68) and I (61) are.
A striking conclusion from this analysis is the value of carbon dioxide -- only 450 ppm, with estimated uncertainty of 100 ppm -- at which the transition occurs from no large ice sheet to a glaciated Antarctica.
This has a clear, strong implication for what constitutes a dangerous level of atmospheric carbon dioxide. If humanity burns most of the fossil fuels, doubling or tripling the preindustrial carbon dioxide level, Earth will surely head toward the ice-free condition, with sea level 75 meters (250 feet) higher than today.
It is difficult to say how long it will take the melting to be complete, but once ice sheet disintegration gets well under way, it will be impossible to stop.
With carbon dioxide the dominant climate forcing, as it is today, it obviously would be exceedingly foolish and dangerous to allow carbon dioxide to approach 450 ppm.
Hansen says that humans now are increasing atmospheric carbon dioxide by 2 ppm per year, by burning fossil fuels -- 67 times more rapid than the fastest fairly recent natural change.
What took nature thousands of years to accomplish, humans are going to do much, much, quicker. Unless we wake up and recognize the danger we're in.