Thursday, September 18, 2014

Over one century of the science of human-caused climate change

How long has science been concerned about greenhouse gases?

Originally posted to Helium Network on Feb. 10, 2013.

Public domain image by NASA. Earth's western hemisphere, as seen from space. © Wikimedia Commons.

The metaphor of the Earth's atmosphere as a greenhouse was born in the nineteenth century when it was discovered that atmospheric gases trap heat.  So-called greenhouse gases — including carbon dioxide, water vapor, and methane — permit short-wave radiation from the sun to pass through to the Earth but prevent long-wave radiation from the Earth from departing into space. (The metaphor is a bit of a misnomer, as part of what makes a greenhouse hot is the heating of the glass itself, whereas clearly Earth is not surrounded by glass.)

All other things being equal, the temperature will rise as more greenhouse gases are added to the atmosphere. This is linked to a complex set of natural processes. For example, carbon dioxide, a byproduct of the combustion of carbon-based fossil fuels like coal and oil, is naturally removed from the atmosphere when it dissolves into the ocean or is absorbed into plants which return the carbon to solid form.

The first scientist to speculate that the carbon dioxide emissions from the burning of fossil fuels could heat the planet was Svante Arrhenius of Sweden in 1896. This is discussed in Spencer Weart's book The Discovery of Global Warming (2008).

In the 1930s, scientists knew that the planet had warmed over recent decades, but they didn't know why. The English scientist Guy Stewart Callendar (1898-1964) said it was caused by the greenhouse effect, but his theory, proposed over three decades after Arrhenius's, was not taken seriously. Callendar's archived correspondence is kept today at the library of the University of East Anglia.

In the 1950s, scientists began to investigate the possibility that carbon dioxide was indeed raising the average global temperature. Finally, in 1960, their results showed that the level of atmospheric carbon dioxide was rising each year and that this could indeed explain the rising global temperatures.

Atmospheric carbon dioxide is measured in parts per million (ppm). Scientists can get air samples from millions of years ago by drilling into glaciers that formed in the eras since the dinosaurs roamed the earth (when there was no ice on the planet). Samples obtained in this way indicate that carbon dioxide levels naturally fluctuated between 200 and 280 ppm for millions of years. However, this level has risen steadily since the beginning of the industrial revolution. Today, in 2013, the levels of atmospheric carbon dioxide are at 394 ppm, according to the advocacy group CO2Now. The advocacy group 350.org recommends 350 ppm or lower as a necessary goal. That was the actual level in 1987.

Scientific issues related to climate change and its effects include the melting of the ice caps and the rising of sea levels; ocean saltiness and circulation patterns; greenhouse gases, measured at the tailpipe or at their natural sources; possibly mitigating factors like aerosols, the ozone layer and its depletion, and clouds; the capability of forests and oceans to store carbon; the archaeological climate record, analyzed through fossil shells or ice bubbles trapped in glaciers; ecosystem observations; temperature changes; and animal behavior trends, including reproduction and migration, as a response to temperature changes.

One of the most complicated issues is feedback loops by which a small amount of warming could generate more warming. For example, if warming causes plants to die, reflective snow to melt, and water to evaporate, then all of these occurrences will contribute to further warming. The resulting warming trend would be largely out of human control.

Individual governments contribute to research. Sam Bodman, then U.S. Deputy Secretary of Commerce, said before a Climate Change Science workshop in Dec. 2002: "Overall, we have spent more than $20 billion since 1990 on climate research — three times as much as any other country." (Bodman later served as U.S. Secretary of Energy from 2005-2009.)

However, because the greenhouse effect concerns the planet as a whole, a full understanding of it would mean nothing less than a full understanding of the planet. Scientists from diverse nations and disciplines have to share information to form a more complete picture.

Periodic reports from the Intergovernmental Panel on Climate Change (IPCC), established by the UN in 1988, synthesize previously published research with this purpose in mind. The IPCC's Fifth Assessment Report (AR5) will be delivered in stages between September 2013 and October 2014.

Famous international climate treaties include the Kyoto Protocol in 1997 and the Copenhagen Accord in 2009.

The greenhouse effect is natural and self-regulating. There is supposed to be some carbon dioxide, water vapor, and methane in the atmosphere. It is what makes the planet warm and habitable. The risk, however, is that industrial contributions of greenhouse gases might tip the natural cycles off balance. Indeed, there is evidence that this is already happening. There might be a threshhold at which the self-regulating patterns are entirely interrupted and derailed. Scientists are not yet sure what that threshold might be, whether tailpipe emissions are likely to bring the world to that point, or indeed whether they already have.

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