Wednesday, September 18, 2013

Rates of change

One common misunderstanding about how the current global warming differs from past episodes of warming is the rate of warming.  In this post, I'll show how the rate over the past 30 years stacks up with two of the better-known rates from geologic history.

Past 30 years (1983-2013) rate ± standard error:
UAH: +0.015379 ± 0.003783ºC per year
GISS: +0.015505 ± 0.002491ºC per year
NCDC: +0.014454 ± 0.002489ºC per year
HadCRUT4: +0.014896 ± 0.002824ºC per year

Depending on the data set, the rate of the last 30 years ranges from 0.014454ºC per year up to 0.015505ºC per year.  When I average the four data sets together then calculate the rate, the result is +0.014692 ± 0.003070ºC per year for the last 30 years.

For the geologic rates, let's start with the most recent and work backwards in time.

Over the 5,000 years since the end of the Holocene Climatic Optimum, the Earth slowly cooled by 0.7ºC (Marcott et al. 2013).  That's an average rate of  -0.00014ºC per year.  The current rate is 104x faster than the rate over the previous 5,000 years.

At the end of the last ice age, a process lasting between 22,000 to 11,000 BP, global temperatures rose an average of 3.5ºC over 8,000 years (Shakun et al. 2012).  That translates to an average of 0.0004375ºC per year, which means the average rate of change over the last 30 years is 33.5x faster than at the end of the last ice age.

During the Paleocene-Eocene Thermal Maximum (55 million years BP), global temperatures rose an average of 6.5ºC over 19,000 years (Cui et al. 2011).  This warming was triggered by a release of an average of 6.2 billion metric tons of CO2 per year.  In contrast, humans released the equivalent of 34.8 billion metric tons of CO2 (9.5 billion metric tons Carbon) in 2011 alone (Quéré et al. 2012).  Not only is the rate of CO2 release greater, the rate of change in temperature over the last 30 years is 43x greater than it was then.

Less well known but a subject of active research is the End-Triassic Mass Extinction 210 million years ago.  This extinction was a time when 50% of species went extinct due to a massive disruption of the carbon cycle and climate.  While I cannot find an estimate of the temperature change, the amount of CO2 has been estimated at 12 trillion metric tons of carbon over a period of 10,000 to 20,000 years (Ruhl et al. 2011), which averages out to between 0.6 billion metric tons and 1.2 billion metric tons per year, far lower than the 9.5 billion metric tons of carbon human activities released in 2011 alone.  The cause of that carbon release is disputed, with Ruhl et al. favoring methane clathrates whereas Blackburn et al. (2013) ascribed it to massive volcanic eruptions triggered as Pangea broke apart.

The lesson from examining geologic history is quite clear.  The current rate of temperature is simply far faster than what has been experienced in the past.  This graph of Holocene temperatures sums up that point nicely:

The only thing standing between us today and the major ecosystem changes, species extinctions, etc noted in the geologic record is time, as the current rate of change is more than fast enough to trigger such events if it continues.  Add in the fact that the Milankovitch cycles, which have been the primary drivers of the Ice Ages, are 6,000 years into a 29,000-year cooling phase (Imbrie and Imbrie 1980) and it's not too hard to see that the current warming is counter to the natural cycle and occurring too fast to be natural.

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