Wednesday, December 2, 2015

A Climate Change Primer

I don't write here much on the subject of climate change. This is in spite of the fact that I am keenly interested in the topic and it was climate change that really got me started blogging. The reason for my reluctance to return to the topic will be obvious to anyone who has been active in the public debate, but given the significance of the 21st Conference of Parties to the United Nations Framework Convention on Climate Change (COP21) being held in Paris, which has been hailed as the greatest gathering of world leaders in history, I thought it was time I returned to the subject and explained in layman's terms what all the fuss is about.

Around eight years ago I was talking to a friend of mine who is a prominent New Zealand scientist about the question of anthropogenic global warming and he urged me to read the 2007 Fourth Assessment Report (AR4) of the UN Intergovernmental Panel on Climate Change (IPCC), which had just been released.

At the time I was pretty neutral on the issue and I wanted to trust the authors of the report - after all, they were supposed to be experts in the field - but as I started to delve into AR4, I began to have some concerns about what I was reading. While I am not a scientist, I studied maths and statistics at university and climate science is more about these fields - analysing past trends and making predictions - than anything else. There is a bit of physics involved and this is entirely incontrovertible - Svante Arrhenius, a Swedish physicist, first suggested in 1896 that increases in atmospheric carbon dioxide (CO2) would lead to global temperature rises. The scientific disagreement (and there really is a lot of scientific disagreement - 97% consensus claims notwithstanding) is really in two areas - the extent to which increases in CO2 will drive a relative increase in global temperatures, and the extent to which carbon emissions from human activity are responsible for the CO2 increases. Let us deal with the second of these two issues first.

Carbon in the atmosphere is measured very accurately these days. The most reliable measurement is done by the Nasa Earth Observatory on the top of Mount Kilauea in Hawaii. It has been taking accurate measurements since the 1950s, during which period atmospheric carbon dioxide has increased from around 320 parts per million to around 400 ppm. Prior to this, the record is less accurate but other sources suggest the atmospheric carbon dioxide content increased from around 270ppm in the mid 19th Century, and some ice-core records suggest CO2 levels rose from as low as 220ppm in the 18th Century (which would indicate the trend started well before there were significant fossil-fuel emissions).

The scientific claim that humans are causing the increase is based on chemical analysis of the composition of the carbon in the atmosphere, specifically the change in the ratio between the quantity of the isotopes of Carbon-12 and Carbon-13. Biological sources such as trees contain more Carbon-12 so a measured decrease in the Carbon-13/Carbon-12 ratio in the atmosphere means the source of the emissions is likely to be biological. A major source of biological carbon emissions is, of course, the coal, oil and gas extracted from the ground and burned by humans, but there are also natural biological emissions from swamps, forest fires and the like. Further evidence that the increase is caused by humans is derived from the fact that the increase in atmospheric carbon dioxide in recent years roughly equates to the sum of human carbon emissions. 

It all seems perfectly logical to assume that mankind is causing the increase in carbon emissions until you consider that carbon in the atmosphere has never been constant and in fact there is strong evidence that atmospheric carbon dioxide levels rose to similar levels to today during earlier periods in the Holocene (up to around 12,000 years ago) and much higher than current levels in prehistoric eras. Some scientists have provided evidence that over long time periods increases and decreases in atmospheric CO2 tend to follow rather than lead corresponding changes in temperatures. Others have provided evidence that the changes in the C-13/C-12 ratio do not correlate with mankind's carbon emissions either geographically (i.e. the greatest increases are not over industrialised areas such as North America and Europe) or with seasonal cycles of human carbon output (e.g. increased fossil fuel use in cold climates during winter).

The other issue is whether anthropogenic carbon emissions will cause the sort of temperature increases that are predicted by some scientists and politicians. This depends on the effect of what physicists call 'forcing'. Scientists agree that increasing carbon dioxide in the atmosphere will not on its own produce an increase in temperatures anything close to the maximum level predicted in the AR4 report (6.6℃ by 2100). This predicted level of increase arises from computer models that multiply the warming effect of carbon dioxide by other factors, the most important of these being an increase in cloud cover. But the effect of increased cloud cover is known to be both positive and negative in terms of temperature increases. Water molecules in the upper atmosphere act as a greenhouse gas but clouds also reduce the amount of the sun's radiation reaching Earth thereby tending to reduce temperatures. Many scientists believe that increases in carbon dioxide will have a diminishing, not an increasing, impact on global temperatures.

The best indication of the likely future increase in temperatures is provided by the past increase. Average global temperatures have increased about 0.76℃ since the mid-19th Century - around 0.05℃ per decade. The rate of increase went up in the last few decades of the 20th Century to about 0.13℃ per decade but temperatures have been largely static since 1998. A reasonable worst case scenario is that the rate of increase in the late 20th Century will soon resume, which would mean we will see a 1℃ increase over the entire 21st Century. To put that in perspective, the total increase in temperatures from the Little Ice Age around the year 1600 to the present day is probably about 2℃. This period has seen the greatest increase in human life expectancy and quality of life in the history of our species, something that some historians ironically credit in part to the warmer climate.

Is this issue, as President Obama and the Pope are claiming, the greatest threat facing mankind? I certainly do not think so. Weapons of mass destruction, epidemic diseases, religious fundamentalism, even collisions with objects from outer space are probably greater threats to our civilisation. So why has this become the biggest issue of our time? Well, answering that question is probably another whole blog post.

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