Global Temperature Trends: 2007 Summation
The year 2007 tied for second warmest in the period of instrumental
data, behind the record warmth of 2005, in the Goddard Institute for
Space Studies (GISS) analysis. 2007 tied 1998, which had leapt a
remarkable 0.2°C above the prior record with the help of the "El Niņo
of the century". The unusual warmth in 2007 is noteworthy because it
occurs at a time when solar irradiance is at a minimum and the
equatorial Pacific Ocean is in the cool phase of its natural El Niņo-
La Niņa cycle.
Figure 1 (Please see:
http://data.giss.nasa.gov/gistemp/20...2007annual.gif )
shows 2007 temperature anomalies relative to the 1951-1980 base period
mean. The global mean temperature anomaly, 0.57°C (about 1°F) warmer
than the 1951-1980 mean, continues the strong warming trend of the
past thirty years that has been confidently attributed to the effect
of increasing human-made greenhouse gases (GHGs) (Hansen et al. 2007).
The eight warmest years in the GISS record have all occurred since
1998, and the 14 warmest years in the record have all occurred since
1990.
Arctic Warmth
The map reveals that the greatest warming has been in the Arctic and
neighboring high latitude regions. Polar amplification is an expected
characteristic of global warming, as the loss of ice and snow
engenders a positive feedback via increased absorption of sunlight.
The large Arctic warm anomaly of 2007 is consistent with observed
record low Arctic sea ice cover in September 2007.
[ . . . ]
Summary
The Southern Oscillation and the solar cycle have significant effects
on year-to-year global temperature change. Because both of these
natural effects were in their cool phases in 2007, the unusual warmth
of 2007 is all the more notable. It is apparent that there is no letup
in the steep global warming trend of the past 30 years (see 5-year
mean curve in Figure 1a).
"Global warming stopped in 1998," has become a recent mantra of those
who wish to deny the reality of human-caused global warming. The
continued rapid increase of the five-year running mean temperature
exposes this assertion as nonsense. In reality, global temperature
jumped two standard deviations above the trend line in 1998 because
the "El Niņo of the century" coincided with the calendar year, but
there has been no lessening of the underlying warming trend.
Global Predictions
The quasi-regularity of some natural climate forcing mechanisms,
combined with knowledge of human-made forcings, allows projection of
near-term global temperature trends with reasonably high confidence.
Prediction for a specific year is a bit hazardous, as evidenced by an
incorrect prediction of record global warmth made by the British
climate analysis group for 2007. Such speculations are useful, as they
draw attention to the mechanisms, and allow testing of understanding.
Presumably part of the basis for their prediction was an assumption of
a continued warming contribution from the 2006 El Niņo. However,
evidence of El Niņo warmth disappeared very early in 2007.
Solar irradiance will still be on or near its flat-bottomed minimum in
2008. Temperature tendency associated with the solar cycle, because of
the Earth's thermal inertia, has its minimum delayed by almost a
quarter cycle, i.e., about two years. Thus solar change should not
contribute significantly to temperature change in 2008.
La Niņa cooling in the second half of 2007 (Figure 2) is about as
intense as the regional cooling associated with any La Niņa of the
past half century, as shown by comparison to Plate 9 in Hansen et al.
(Hansen et al. 1999) and updates to Plate 9 on the GISS web site.
Effect of the current La Niņa on global surface temperature is likely
to continue for at least the first several months of 2008. Based on
sequences of Pacific Ocean surface temperature patterns in Plate 9, a
next El Niņo in 2009 or 2010 is perhaps the most likely timing. But
whatever year it occurs, it is a pretty safe bet that the next El Niņo
will help carry global temperature to a significantly higher level.
Competing with the short-term solar and La Niņa cooling effects is the
long-term global warming effect of human-made GHGs. The latter
includes the trend toward less Arctic sea ice that markedly increases
high latitude Northern Hemisphere temperatures. Although sea ice cover
fluctuates from year to year, the large recent loss of thick multi-
year ice implies that this warming effect at high latitudes should
persist.
Based on these considerations, it is unlikely that 2008 will be a year
with truly exceptional global mean temperature. These considerations
also suggest that, barring the unlikely event of a large volcanic
eruption, a record global temperature clearly exceeding that of 2005
can be expected within the next 2-3 years.
[ . . . ]
http://data.giss.nasa.gov/gistemp/2007/