[Terrapreta] A reasonable explanation?

[MFH]

Warming trend has not been reversed


 
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David Karoly | April 29, 2008 

THE opinion piece by Phil Chapman ("Sorry to ruin the fun, but an ice age
cometh", Opinion, April 22) warns of an approaching ice age but contains a
number of factual errors, misleading statements and incorrect conclusions.

Chapman reports global average temperature cooled by 0.7C in 2007 and says:
"If the temperature does not soon recover, we will have to conclude that
global warming is over." 

It is true that global data sets show a pronounced cooling from January2007
to January 2008 of slightly less than 0.7C. It is an error to state, as
Chapman does, that this is unprecedented, as similar dramatic falls occurred
from 1998 to 1999, and from 1973 to 1974. It should also be noted that the
global average temperature has warmed substantially, by about 0.3C from
January 2008 to March 2008. In addition, the annual average temperature for
2007 was within 0.1C of the average temperature in 2006 and 2005; no
dramatic cooling there. 

So what caused this rapid cooling during 2007, and also from 1998 to 1999,
and from 1973 to 1974? What was common to all those periods? In each case,
the common factor was a rapid change from El Nino to La Nina conditions,
from warm temperatures in the eastern equatorial Pacific Ocean to cold
temperatures in the same region, which has a significant effect on global
climate patterns and global average temperature. La Nina is associated with
below-normal global average temperature, and because of its influence, 2008
is likely to be about 0.3C cooler than the average of the previous few
years. 

Chapman did not consider La Nina as a cause of the cooling in 2007 and
instead linked it to the minimum in the 11-year cycle in sunspot numbers:
"The first sunspot appeared in January this year and lasted only two days. A
tiny spot appeared last Monday but vanished within 24 hours. Another little
spot appeared this Monday." 

I don't know where these sunspot numbers came from but they are in error.
The best source of data for present sunspot numbers is the World Data Centre
for Solar Terrestrial Physics at the National Geophysical Data Centre in
Boulder, Colorado. According to it, the average number of sunspots a day
last January was 3.4, followed by 2.1 in February and 9.3 in March. The
minimum was in October2007. 

So, are variations in global average temperature directly related to sunspot
numbers on a monthly, annual or decadal timescale? 

Certainly not on a monthly timescale and the effect, if any, on a
year-to-year timescale is very small, as can be found by correlating the
variations of global average temperature on monthly or annual timescales
with the sunspot numbers. Any relationship between sunspot numbers and
global average temperatures is much, much smaller than the clear
relationship between inter-annual variations of equatorial Pacific Sea
surface temperatures and global average temperatures, showing the effect of
the El Nino-La Nina cycle. 

While those errors are bad enough, the main flaw in Chapman's opinion is
trying to infer long-term climate trends from short-term (one year)
variations of global temperature. It is well known (among climate
scientists) that there are large inter-annual variations of global
temperature caused by a number of factors, including El Nino, big volcanic
eruptions, or just the chaotic variability of the climate system. It is not
possible to make conclusions about long-term climate trends from
inter-annual climate variations. Many lines of evidence support the
conclusion reached last year by the Intergovernmental Panel on Climate
Change that "warming of the climate system is unequivocal", referring to
changes over the past 100 years. Even when we consider only the global
average temperature during La Nina episodes, such as the present cool
period, we find that we are experiencing the warmest global temperature of
any strong La Nina episode in the past 100 years, again showing clear
long-term global warming. 

Most of the increase in global average temperature over the past 50 years is
due to the increase in greenhouse gases in the atmosphere. This long-term
increase in global average temperature will continue throughout the 21st
century because of further increases in greenhouse gases. Yes, there will be
year-to-year natural climate variations, with some colder years, but the
long-term warming trend will continue. 

An ice age is definitely not going to occur in the 21st century. Instead, we
will all need to make very large reductions in emissions of greenhouse gases
if we are to minimise dangerous anthropogenic climate change. 

David Karoly is a professor in the University of Melbourne's school of earth
sciences and a member of the Wentworth Group of Concerned Scientists.

 

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