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	It's the sun
	Climate's changed before
	There is no consensus
	Surface temp is unreliable
	Models are unreliable
	It's cooling
	Ice age predicted in the 70's
	Al Gore got it wrong
	We're heading into an ice age
	CO2 lags temperature
	View All Arguments...

Latest Posts

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• Did global warming stop in 1998?
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• Cartoon about global warming alarmism
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• The Mystery of the Vanishing Ocean Heat
• Global warming stopped in 1981... no, wait! 1991!
• Determining the long term solar trend
• Comparing IPCC projections to observations
• Climate change on Mars
• Global cooling: the new kid on the block
• What 1970's science said about global cooling
• Has solar cycle 24 begun?
• Empirical evidence for positive feedback
• Al Gore and Dr Thompson's thermometer
• Does model uncertainty exagerate global warming projections?
• Are we heading into a new Little Ice Age?
• Did global warming cause Hurricane Katrina?
• What does CO2 lagging temperature mean?
• Svensmark and Friis-Christensen rebut Lockwood's solar paper
• Evaporating the water vapor argument
• They predicted an ice age in the 70's
• Do 500 scientists refute anthropogenic global warming?
• Climate change on Jupiter
• Schulte vs Oreskes on consensus, Round 2
• Do growing glaciers disprove global warming?

If scientists can't predict weather, how can they predict long term climate?

What the science says...

This argument betrays a misunderstanding of the difference between weather, which is chaotic and unpredictable and climate which is weather averaged out over time. While you can't predict with certainty whether a coin will land heads or tails, you can predict the statistical results of a large number of coin tosses. Or expressing that in weather terms, you can't predict the exact route a storm will take but the average temperature and precipitation will result the same for the region over a period of time.

Climate prediction is a difficult and ever refining art. There's the problem that future behaviour of the sun is very difficult to predict. Similarly, short term perturbations like El Nino or volcanic eruptions are difficult to model. Nevertheless, climate scientists have a handle on the major drivers of climate.

James Hansen's 1988 climate predictions

Way back in 1988, James Hansen projected future temperature trends (Hansen 1988). Those initial projections show remarkable agreement with observation right to present day (Hansen 2006). Hansen even speculated on a volcanic eruption in 1995 but missed the date by a few years (we'll cut him some slack there).

Figure 1: Hansen's model projections (green, blue, purple) compared to observations (red and blue).

Hansen's Scenario B (described as the most likely option and in hindsight, the one that most closely matched the level of CO2 emissions) shows close correlation with observed temperatures. In fact, Hansen overestimated future CO2 levels by 5 to 10% so if his model was given the correct forcing levels, the match would be even closer. There are deviations from year to year but this is to be expected. The chaotic nature of weather will add noise to the signal but the overall trend is predictable.

Modelling the aftermath of the Mount Pinatubo volcanic eruption

A good example of this is volcanic eruptions. When Mount Pinatubo erupted in 1991, it gave NASA the opportunity to predict the new few years and test out how successfully their model factored in sulfate aersol forcings. Not only was the subsequent global cooling of about 0.5 °C accurately forecast soon after the eruption, but the radiative, water-vapor, and dynamical feedbacks included in the models were quantitatively verified (Hansen 2007).

Figure 2: Green and purple are observed data, red is model output. Graph courtesy of NASA.

Comparing IPCC projections to observations

Recent Climate Observations Compared to Projections (Rahmstoorf 2007) compared 2001 IPCC projections of global temperature change (coloured dotted lines) with observations from HadCRUT (blue) and NASA GISS data (red). The thin lines are the observed yearly average. The solid lines are the long term trends, which filter out short term weather fluctuations.

Figure 3: courtesy of Tamino: Solid blue and red lines are trends from GISS and HadCRU data, dashed lines are IPCC projections.

It's immediately apparent the IPCC underestimated temperature rise with observations warmer than all projections (but inside the grey uncertainty area). The paper proposes several possible reasons for the difference. One is intrinsic internal variability which is possible over such a short period. Another candidate is climate forcings other than CO2 such as aerosol cooling being smaller than expected.

A third candidate is an underestimation of climate sensitivity. The IPCC assumed a climate sensitivity of 3°C with an uncertainty range between 1.7° to 4.2°C (this is indicated in the grey area of Figure 2). However, there are a number of positive feedbacks in the climate system that are poorly understood and hence not given much influence in IPCC models. Add to this the fact that model uncertainty is inherently skewed towards greater sensitivity. My guess is higher climate sensitivity is part of the story but not all. More on IPCC's 2001 projections...

Other results successfully predicted and reconstructed by models

• Cooling of the stratosphere
• Warming of the lower, mid, and upper troposphere
  
• Warming of ocean surface waters (Cane 1997)
• Trends in ocean heat content (Hansen 2005)
  
• An energy imbalance between incoming sunlight and outgoing infrared radiation (Hansen 2005)
  
• Amplification of warming trends in the Arctic region (NASA observations)