According to NASA scientists (PDF):
Through the first 11 months, 2007 is the second warmest year in the period of instrumental data, behind the record warmth of 2005, in the Goddard Institute for Space Studies (GISS) analysis. 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 has entered the cool phase of its natural El Niño — La Niña cycle.
… barring the unlikely event of a large volcanic eruption, a record global temperature exceeding that of 2005 can be expected within the next 2-3 years.
Figure (a) Annual surface temperature anomaly relative to 1951-1980 mean, based on surface air measurements at meteorological stations and ship and satellite measurements of sea surface temperature; the 2007 point is the 11-month anomaly. [Green error bar is estimated 2σ uncertainty …]
Even an “unusually cold” December would only drop 2007 to the third warmest year ever. NASA points out:
The six warmest years in the GISS record have all occurred since 1998, and the 15 warmest years in the record have all occurred since 1988.
Anyone notice a trend? And the most warming is far from the urban heat islands of major cities:
… the greatest warming has been in the Arctic. 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 observations of record low Arctic sea ice cover in September this year.
But couldn’t this all be the sun going through a phase of high solar radiation, a favorite explanation of those who deny that human-generated greenhouse gases are the primary cause of warming? No. As NASA explains:
The sun is another source of natural global temperature variability. Figure 3, based on an analysis of satellite measurements by Richard Willson, shows that 2007 is at the minimum of the current 10-11 year solar cycle. Another analysis of the satellite data (not illustrated here), by Judith Lean, has the 2007 solar irradiance minimum slightly lower than the two prior minima in the satellite era.
This cyclic solar variability yields a climate forcing change of about 0.3 W/m2 between solar maxima and solar minima … Several analyses have extracted empirical global temperature variations of amplitude about 0.1°C associated with the 10-11 year solar cycle, a magnitude consistent with climate model simulations …
The solar minimum forcing is thus about 0.15 W/m2 relative to the mean solar forcing. For comparison, the human-made greenhouse gas climate forcing is now increasingly at a rate of about 0.3 W/m2 per decade [PDF]. If the sun should remain “stuck” in its present minimum for several decades, as has been suggested in analogy to the solar Maunder Minimum of the seventeenth century, that negative forcing would be balanced by a 5-year increase of greenhouse gases. Thus such solar variations cannot have a substantial impact on long-term global warming trends.
Not only are we in a solar minimum, we are also in a cool phase for the Pacific Ocean:
The cooler than normal equatorial region to the west of South America reflects the building La Niña phase of the Southern Oscillation. In the La Niña phase of the El Niño-La Niña cycle the equatorial winds in the Pacific Ocean blow with stronger than average force from the east, driving warm surface waters toward the Western Pacific. This induces an upwelling of cold deep water near Peru, which then spreads westward along the equator.
What does this all mean?
The natural variations of the Southern Oscillation and the solar cycle thus have minor but not entirely insignificant effects on year-to-year temperature change. Given that both of these natural effects were in their cool phases in 2007, it makes the unusual warmth this year all the more notable. It also suggests that, barring the unlikely event of a large volcanic eruption, a record global temperature exceeding that of 2005 can be expected within the next 2-3 years.
As you may recall, a Science magazine article from August predicted “at least half of the years after 2009 [will] exceed the warmest year currently on record.”
Perhaps these record-breaking temperatures will finally move this country to action.