We all know that many things affect global temperature. Some, like greenhouse gases, cause long-term changes and can create significant trends. Others fluctuate but don’t really go anywhere over the long term, so they cause temperature fluctuations but don’t create long-term trends.
At least two such fluctuating influences have been identified. One is solar variation. The energy output of the sun is variable, especially showing a cyclic variation with the roughly 11-year solar cycle. Another is the el Nino Southern Oscillation (ENSO), which when high warms the planet and when low cools it. It too fluctuates up and down, but hasn’t exhibited long-term trends which could account for the trend we observe in global temperature. You might wonder, what has been the short-term impact of these two influences recently? Let’s look at their temperature influence over a recent 10-year period, from January 2002 through December 2011.
Their temperature influence can be estimated by multiple regression of temperature as a function of long-term trend and exogenous factors. This enables us not only to correct the temperature data for such things as solar variation and ENSO, it also enables us to estimate their impact.
I recently re-computed the regression to update it through December 2011, using TSI (total solar irradiance) to represent solar activity and MEI (multivariate el Nino index) to represent ENSO. Here’s the influence of solar variation on global surface temperature (from NASA GISS) over the last 10 years:
Declining solar activity over this 10-year period has caused a drop in temperature, which certainly is not evidence that global warming has stopped or even slowed. But it has introduced a rather sizeable trend of -0.05 deg.C/decade over that time span — one which fake skeptics conveniently forget to mention when they discuss recent temperature trends. It’s one of the reasons that 10 years is far too little time to estimate long-term temperature trends reliably — a fact which fake skeptics also conveniently omit to discuss.
Despite the sizeable (but temporary!) trend caused by solar variation, then trend due to ENSO is even larger:
Note that the trend over these 10 years is dominated by starting with sustained el Nino conditions and ending with two major la Nina events of 2008 and 2011. Therefore the temporary influence of ENSO has caused a 10-year trend of -0.09 deg.C/decade.
If we include all the exogenous factors used here, then we must also add volcanic activity into the mix. It has actually caused a warming trend, but one so slight that it has only a tiny impact on the net temporary trend. Here’s the total effect:
Over the 10-year period from 2002 through the end of 2011, exogenous factors created a temporary trend of -0.13 deg.C/decade. That’s substantial — if not for the global warming due to greenhouse gases, we would have observed a sizeable global cooling trend.
Temperature in the troposphere (the lower layer of the atmosphere, where most of our weather takes place) is even more strongly influence by these factors than surface temperature. Here’s the estimated influence of solar variation on tropospheric temperature according to UAH:
Solar variation introduced a temporary trend of -0.06 deg.C/decade. The influence of ENSO is even stronger:
The temporary trend in UAH temperature due to ENSO is -0.16 deg.C/decade. If we include volcanic activity as well, we again find its influence over this 10-year period is tiny, and the total influence of all three factors is:
The temporary trend in UAH temperature due to these fluctuating factors is a whopping -0.21 deg.C/decade.
These known factors do indeed have a sizeable, but only temporary, influence on temperature. So sizeable, that we would indeed have witnessed substantial global cooling over these 10 years, except for the also-substantial global warming caused by … you guessed it … human activity.