Last night, a reader’s comment caused to me provide an explanation of the greenhouse effect and how this shows us that human activity is primarily driving climate change. I was asked to repost that comment here.
The scientific process is simple: state a hypothesis, test it, and analyze the results. There are many hypotheses involved in climate change, but let’s start with the one most fundamental to human causation. We know that so-called greenhouse gases trap heat in the Earth’s atmosphere; if they didn’t, the Earth would resemble Mars and life would probably never have evolved (or at least, never gotten beyond single-celled extremophiles living in volcanic vents). The greenhouse effect is the only reason I’m sitting here typing this response; it’s supposed to occur.
But let’s prove that this is true. The Vostok ice core (perhaps the most famous of the ice cores drilled so far) gives us measures of temperature and carbon dioxide concentration for the past 420,000 years, a period twice as long as the age of our species and involving multiple major fluctuations in both metrics (natural climate change). In that period, the highest concentration of carbon dioxide occurred about 320,000 years ago, when it reached 300 parts per million (ppm). This was also one of the hottest period in the record. Every fluctuation in carbon dioxide levels is matched by a fluctuation in temperature; both go up at the same time, and both go down at the same time.
So here is our testable hypothesis: an increase in the concentration of greenhouse gases will lead to an increase in the heat trapped by the atmosphere. The current concentration of carbon dioxide is 390 ppm, the highest in 800,000 years (known from other ice cores). In 1832, around the start of the Industrial Revolution, the concentration was 284 ppm.
We have accurate temperature records based on meteorology, rather than ice cores, from 1880 onward. The 10 warmest years on record for global temperature in that period (averaging out land and ocean temperatures) were: 2005, 1998, 2003, 2002, 2006, 2009, 2007, 2004, 2001 and 2008. We have a correlation for this time period: increasing carbon dioxide levels matched with increasing temperature levels.
Now, most carbon dioxide emissions come from natural sources, which are offset by natural “sinks,” processes which remove carbon dioxide from the atmosphere, such as photosynthesis or absorption into the ocean. However, this is a very fragile balance; a shift in carbon dioxide levels leads to a warming period or an ice age. These are natural occurrences. But humans are producing massive amounts of carbon dioxide; in 2008, it was 31.8 gigatons. That’s 31,800,000,000 tons. Human production of carbon dioxide has overwhelmed these natural processes, leading to the observed and measured increase in carbon dioxide concentration in the atmosphere.
According to the Vostok ice core data, a concentration of 300 ppm accounted for a temperature increase of about 3 degrees Celsius. This of course is not immediate; even a century is less than the blink of an eye compared to a 4.5 billion year old Earth. But the scientists are clear: 2 degrees Celsius means the ice caps are done for. It’s pretty simple really: water is in a solid state when 0 degrees Celsius, but is liquid at 1 degree.
One effect of the absorption of carbon dioxide in the ocean is that the ocean’s acidity will increase (which can also be phrased as having its pH decrease). This is another hypothesis we can make and test. Bear in mind that a pH scale goes from 0 to 14. Between 1751 and 1994, the pH of the ocean has decreased by 0.075. The projection for 2050 is a total decrease of 0.230.
All of the available data supports the hypothesis that human emission of greenhouse gases has led to climate change.
This is really only the proverbial tip of the iceberg; I haven’t even mentioned receding glaciers and ice caps, rising water levels, rapidly shifting weather patterns, increased incidence of severe storms, increased incidence of disease, the spread of disease-carrying insects, the thinning of the atmosphere, the increasing level of radiation making it through our thinning atmosphere, the appalling extinction rate, the rapidly decreasing fresh water supply, the increased incidence of both flooding and drought and the ensuing crop failures, and, most frightening of all, the thawing of the tundra and the release of the immense quantities of carbon dioxide and methane that had been previously been locked underground. I’m sure I’ve left items off that list.
If you want more information, I’d direct to you a paper published in Nature in 2008, which examined data from 80 studies that involved at least 20 years of data collection. As a refresher in Greek, “anthropogenic” means “derived from human activities.”
Rosenzweig C et al. 2008. Attributing physical and biological impacts to anthropogenic climate change. Nature 453(15): 353-358.