August 02, 2006
Global warming-7: The current status of the scientific consensus
So what is the scientific consensus about the answers to the key questions concerning global warming?
The British magazine New Scientist gives a review of the state of affairs concerning climate change, along with a handy summary sheet of the main points, and the Intergovernmental Panel on Climate Change (IPCC) report (thanks to Brian Gray of the Kelvin Smith Library who runs the blog e3 Information Overload for the link) provides more detailed information. Here are some tentative answers to the five key questions I raised in a previous post.
1. Is warming occurring? In other words, are average temperatures rising with time?
Here we have to distinguish between the more recent period (starting in 1861) when we have direct measurements of temperature and the prior periods, for which we have to infer temperatures using proxy measures such as using tree rings or bubbles trapped in ice cores that date back 750,000 years.
For the recent past, the IPCC report says that "The global average surface temperature has increased by 0.6 ± 0.2°C since the late 19th century".
For the period prior to that, the report says "It is likely that the rate and duration of the warming of the 20th century is larger than any other time during the last 1,000 years. The 1990s are likely to have been the warmest decade of the millennium in the Northern Hemisphere, and 1998 is likely to have been the warmest year."
2. If so, is it part of normal cyclical warming/cooling trends that have occurred over geologic time or is the current warming going outside those traditional limits?
Some skeptics have pointed to relative warm periods associated with the 11th to 14th centuries, and relative cool periods associated with the 15th to 19th centuries in the Northern Hemisphere as evidence that the kinds of warm temperatures we have witnessed recently are part of global cyclical patterns. However the IPCC reports says that "evidence does not support these “Medieval Warm Period” and “Little Ice Age” periods, respectively, as being globally synchronous." In other words, these were likely regional phenomena.
If we go back even further the report says that "It is likely that large rapid decadal temperature changes occurred during the last glacial and its deglaciation (between about 100,000 and 10,000 years ago), particularly in high latitudes of the Northern Hemisphere. In a few places during the deglaciation, local increases in temperature of 5 to 10°C are likely to have occurred over periods as short as a few decades. During the last 10,000 years, there is emerging evidence of significant rapid regional temperature changes, which are part of the natural variability of climate."
So while rapid localized changes in temperature have occurred, there is little evidence that these were global in scope.
But there are also suggestions that temperature swings in the past may have been greater than originally thought.
3. Are the consequences of global warming such that we can perhaps live with them (slightly milder winters and warmer summers) or are they going to be catastrophic (causing massive flooding of coastal areas due to rising ocean levels, severe droughts, blistering heat waves, total melting of the polar regions, widespread environmental and ecological damage)?
The answer to these important questions, of course, depend on projections for the future which in turn depend on what actions are taken. The IPCC report outlines possible scenarios here. But some things, such as the reductions in the polar ice caps and snow cover generally are already visible.
One of the most dramatic consequences of snow and glacier melting is a rise in sea levels. It is estimated that a 30 cm (one foot) rise in sea levels results in shorelines receding by 30 meters. Some recent studies suggest that the IPCC report estimates of possible rise in sea levels were low, and more recent estimates are that sea levels could rise by six feet, which would result in massive flooding of highly populated areas the world over. Again, there is limited data so these are still rough estimates. But to my mind, the state of the large ice and snow areas (the polar caps, Greenland, glaciers, and mountain tops) are things that we should watch carefully, and the signs there are not good.
4. How reliable are the theories and computer models that are being used study this question?
The IPCC report points out that "The basic understanding of the energy balance of the Earth system means that quite simple models can provide a broad quantitative estimate of some globally averaged variables." But only numerical models can provide the kinds of detailed quantitative projections into the future that we need in order to make informed decisions. "The complexity of the processes in the climate system prevents the use of extrapolation of past trends or statistical and other purely empirical techniques for projections." In other words, just having data about the past is insufficient to project to the future. We also need computer models based on the science and mathematics of climate change. "Climate models can be used to simulate the climate responses to different input scenarios of future forcing agents. . .Similarly, projection of the fate of emitted CO2. . .and other greenhouse gases requires an understanding of the biogeochemical processes involved and incorporating these into a numerical carbon cycle model." (For details on how the computer models used to predict future trends in climate work, see here.)
The IPCC report concludes that "In general, [the computer models] provide credible simulations of climate, at least down to sub-continental scales and over temporal scales from seasonal to decadal. Coupled models, as a class, are considered to be suitable tools to provide useful projections of future climates."
5. What are the causes of global warming? Is human activity responsible and can the process be reversed?
Several of the greenhouse gases that influence global temperatures, referred to as "climate forcing agents" (carbon dioxide, methane, nitrous oxide) have recently shown dramatic increases in concentrations in the atmosphere. This graph is perhaps the one that alarms me the most.
These sharp increases in greenhouse gas concentrations are clearly correlated with rapid increases in the rate of industrialization and energy consumption within the two last centuries. It seems to me that while individual changes in behavior (such as using less stuff and reusing and recycling more) are important, they must be accompanied by concerted international governmental actions to reverse the trends.
We have a precedent for this kind of concerted international action to solve an important environmental problem. Recall the recent time when there was concern that the ozone layer was being damaged by the extensive use of chlorofluorocarbons (CFCs). International action led to the complete ban on its use worldwide. Now there is some good news.
While ozone degradation continues despite global bans on ozone-depleting pollutants imposed more than a decade ago, the rate has slowed markedly enough in one layer of the atmosphere that scientists believe ozone could start to be replenished there within several years.
"There is compelling evidence that we are seeing the very first stages of ozone recovery in the upper atmosphere," said Michael Newchurch, an atmospheric chemist with the National Space Science and Technology Center at the University of Alabama in Huntsville.
Evidence suggests that international efforts to reduce chlorofluorocarbon (CFC) pollution are working.
Of course, greenhouse gases are produced by a much more extensive and powerful group of industries than those producing ozone depleting ones, and require greater changes in our own lifestyles. So achieving international cooperation on this will not be easy, as the difficulties implementing the Kyoto treaty suggests. That treaty committed industrialized nations to commit to reducing their emissions of greenhouse gases within the next decade to a level of about 5% below their 1990 levels. Although the US produces about 36% of the world's output of greenhouse gases (the largest single producer), George W. Bush said in 2001 that the US would not sign the treaty.
Next: The danger of complacency
POST SCRIPT: And sure enough, right on cue. . .
Just last week, I said that the lack of public understanding that climate questions such as global warming only deal with averages over long times and large areas inevitably lead to people drawing the wrong conclusions from short term fluctuations.
Sure enough, yesterday's Plain Dealer has the following letter to the editor:
We constantly are subjected to news about the coming devastating effects of global warming, which includes the recent story on how it is going to dramatically change Lake Erie and its shoreline. So it's a bit perplexing to me to see in my most recent FirstEnergy electric bill that during my past 30-day billing cycle, the average temperature in Cleveland was 69 degrees, versus 72 degrees last year. Now, if we are to believe the global-warming doomsayers, a three-degree swing in temperature is cataclysmic. So when will The Plain Dealer begin printing articles about how Cleveland is at risk of entering an ice age if we don't change our behavior?
Why does the Plain Dealer even print such nonsense? Either they know it is flat out wrong, which means they are deliberately propagating erroneous information, or even the editors don't know the basics about climate. I don't know which is more disturbing.