By Dr. Roy Spencer
A simple climate model forced by satellite-observed changes in the Earth’s radiative budget associated with the Pacific Decadal Oscillation is shown to mimic the major features of global average temperature change during the 20th Century - including two-thirds of the warming trend. A mostly-natural source of global warming is also consistent with mounting observational evidence that the climate system is much less sensitive to carbon dioxide emissions than the IPCC’s climate models simulate.
As Joe D’Aleo and others have pointed out for years, the Pacific Decadal Oscillation (PDO) has experienced phase shifts that coincided with the major periods of warming and cooling in the 20th Century. The pre-1940 warming coincided with the positive phase of the PDO; then, a slight cooling until the late 1970s coincided with a negative phase of the PDO; and finally, the warming since the 1970s has once again coincided with the positive phase of the PDO.
Since this timing between the phase of the PDO and periods of warming and associated climate change seems like more than mere coincidence, I asked the rather obvious question: What if this known mode of natural climate variability (the PDO) caused a small fluctuation in global-average cloud cover? Such a cloud change would cause the climate system to go through natural fluctuations in average temperature for extended periods of time. The IPCC simply assumes that this kind of natural cloud variability does not exist, and that the Earth stays in a perpetual state of radiative balance that has only been recently disrupted by mankind’s greenhouse gas emissions. (This is an assumption that many of us meteorologists find simplistic and dubious, at best.)
I used a very simple energy balance climate model, previously suggested to us by Isaac Held and Piers Forster, to investigate this possibility. In this model I ran many thousands of combinations of assumed: (1) ocean depth (through which heat is mixed on multi-decadal to centennial time scales), (2) climate sensitivity, and (3) cloud cover variations directly proportional to the PDO index values. In effect, I asked the model to show me what combinations of those model parameters yielded a temperature history approximately like that seen during 1900-2000.
The PDO-only” indeed mimics the main features of the behavior of global mean temperatures during the 20th Century—including two-thirds of the warming trend. If I include transient CO2 forcing with the PDO-forced cloud changes (PDO+CO2), then the fit to observed temperatures is even closer. These recent satellite measurements - even though they span less than 6 years—support the Pacific Decadal Oscillation as a potential major player in global warming and climate change.
The evidence continues to mount that the IPCC models are too sensitive (produce too much global warming). If climate sensitivity is indeed considerably less than the IPCC claims it to be, then increasing CO2 alone can not explain recent global warming. The evidence presented here suggests that most of that warming might well have been caused by cloud changes associated with a natural mode of climate variability: the Pacific Decadal Oscillation.
I am posting this information in advance of publication because of its potential importance to pending EPA regulations or congressional legislation which assume that carbon dioxide is a major driver of climate change. Since the news media now refuses to report on peer-reviewed scientific articles which contradict the views of the IPCC, Al Gore, and James Hansen, I am forced to bypass them entirely. We need to consider the very real possibility that atmospheric carbon dioxide - which is necessary for life on Earth and of which there is precious little - might well be like the innocent bystander who has been unjustly accused of a crime based upon little more than circumstantial evidence.
(Read more with graphs at icecap.us)