Thursday, February 26, 2015

A Climate Scientist's Summary of the State-of-the-Climate Science

Dr. Judy Curry is a climate professor at Georgia Tech University. Two days ago, she published a summary of what we know and don't know about the state of the climate. The article appeared on her blog and, with permission, I am reproducing it for my readers. What you see in blue type are her words. I inserted the phrase in brackets.

Climate science is complicated and can be confusing. But the confusion is exacerbated by politicization of the science and also misleading communication by the media. The recent Sense of the Senate Resolution illustrates the problem.
  • “Climate change is real and not a hoax” (98-1)
  • “Climate change is real; and human activity significantly contributes to climate change.” (50-49)
The senate resolutions highlight the differences and confusion between the scientific versus the political definitions of climate change. The scientific definition states that climate change can be due to natural processes OR persistent human caused changes. The political definition is that climate change is caused by humans. The UN Framework Convention on Climate Change established the political definition in the 1990’s.
The political definition effectively defines naturally caused climate change out of existence. However, natural climate change versus human caused climate change is at the heart of the scientific debate. My remarks today will be directed at pointing out the importance of natural climate variability.
Disagreement. So, what do climate scientists agree on? Scientists agree that
  • Surface temperatures have increased since 1880
  • Humans are adding carbon dioxide to the atmosphere
  • Carbon dioxide and other greenhouse gases have a warming effect on the planet
However there is considerable disagreement about the most consequential issues:
  • Whether the warming since 1950 has been dominated by human causes
  • How much the planet will warm in the 21st century
  • Whether warming is ‘dangerous’
  • And whether we can actually do anything to prevent climate change
Why do scientists disagree? There are a number of reasons:
  • Insufficient observational evidence
  • Disagreement about the value of different types of evidence
  • Disagreement about the appropriate logical framework for linking and assessing the evidence
  • Assessments of areas of ambiguity & ignorance
  • And finally, the politicization of the science can torque the science in politically desired directions.
Uncertainty and disagreement drive scientific progress. However, when a scientific issue becomes politicized, and scientists attempt to speak consensus to power, then a scientific discussion of uncertainties is regarded as an undesirable political act.
Wicked vs tame problem. Another source of confusion is oversimplifying both the climate change problem and its solution. The UN Framework Convention and the Obama Administration seem to view climate change as a ‘tame problem’, where we clearly understand the problem and have identified the appropriate solutions.
I view the climate change problem very differently, as a ‘wicked mess’. A wicked problem is complex with dimensions that are difficult to define and changing with time. A mess is characterized by the complexity of interrelated issues, with suboptimal solutions that create additional problems.
You find what you shine a light on. The politicization of climate science, and effectively defining natural climate variability out of the public dialogue, has had a very unfortunate impact on the progress of climate science. Have you heard the story about the drunk searching for his lost keys under a streetlight, since that is the only place where he can see anything? Well something similar has been happening with climate science. You find what you shine a light on.
Motivated by the UN Framework Convention and the Intergovernmental Panel on Climate Change and government funding, climate scientists have been focusing primarily on greenhouse gases and to a lesser extent other anthropogenic ["human caused"] factors. Other factors important for understanding climate variability have been relatively neglected, I have highlighted long-term ocean oscillations and solar indirect effects, since I think that these are potentially very important on decadal to century timescales.
Global surface temperatures. This figure shows the global surface temperature anomalies since 1850. We see a substantial temperature increase from 1910-1940, then a period of weak cooling from 1940 to the late 1970s, then a sharp increase since the late 1970’s until the 21st century when the temperatures are flat.
So what is causing the warming? The recent IPCC AR5  concluded: It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by [humans]. The best estimate of the human induced contribution is similar to the observed warming over this period. The IPCC does NOT have a consistent or convincing explanation for the large warming between 1910 and 1940, the cooling between 1940 and 1975, and the flat temperatures in the 21st century. Until the IPCC is able to explain these variations, I find their high confidence that humans have caused virtually all of the warming since 1950 to be unconvincing.
Last 350 years. So, how unusual is the warming since 1950? The longest temperature record in the world is the Central England Temperature, that goes back to 1660. You see a long term warming trend, but according to the Intergovernmental Panel on Climate Change, only the warming since 1950 is attributed to humans. Note in particular the sharp warming from 1690 to 1740 and 1820-1840. We can’t infer anything about global temperature variations from one location, but the Central England Temperature record serves to illustrate the magnitude of natural climate variability.
Last 2000 years.  Using paleoclimate proxies such as tree rings and ice cores, attempts have been made to reconstruct the hemispheric temperature record for the past 2000 years. Unfortunately, these proxies can’t resolve variations shorter than 50 years. You may have heard of the hockey stick, made famous by Al Gore’s movie, which showed that climate for the past 1000 years was essentially flat, until the 20th century. However, recent research shows much greater variability and uncertainty in these paleoclimate reconstructions. Since 1600, you see a general warming trend. A warmer period around 1000 AD is evident, the so-called medieval warm period. There is a great deal of uncertainty in these analyses, leaving open the question as to whether the warming since 1950 has been unusual.
Hiatus. Lets take a closer look at the recent flat period, which is referred to as the warming ‘pause’ or ‘hiatus’. There was a big warm spike in 1998 from a super El Nino; since then the temperatures have been pretty flat. 2014 was a warm year, tied with several other years for the warmest in the record. Clearly there is a lot of year-to-year variability; why does this pause since 1998 matter? Well, the IPCC Fourth Assessment Report in 2007 stated that surface temperature was expected to increase by 0.2C per decade in the early 21st century. This warming has clearly not been realized.
Significance of the hiatus.  The growing divergence between models & observations raises some serious questions:
  • Are climate models too sensitive to carbon dioxide?
  • Is modeled treatment of natural climate variability inadequate?
  • Are model projections of 21st century warming too high?
Consensus view. The issue of greatest concern is how the climate will evolve during the 21st century. There are two different perspectives on this. The first perspective is that of the Intergovernmental Panel on Climate Change. This figure is from the recent 5th Assessment Report, which projects continued warming. The IPCC cites ‘expert judgment’ as the rationale for lowering the projections (indicated by the red hatching), to account for the apparent oversensitivity of the models. With regards to the ‘pause’, the IPCC expects that it will end soon, with the next El Nino
Natural variability. The other perspective emphasizes natural variability, with the following implications for the future:
  • Our understanding of circulation regimes in the Atlantic and Pacific Oceans suggest that the ‘pause’ will continue at least another decade, perhaps into the 2030’s
  • Climate models are too sensitive to human forcing; 21st century warming will be on the low end of IPCC projections (or even below)
  • Solar variations & volcanoes are a wild card. Some scientists are predicting solar cooling in the near term
  • And finally, we can’t rule out unforeseen surprises. An example of an unforeseen surprise was the warming hiatus in the early 21st century.
Time will tell which of these two views is correct.
I completely agree that forecasting the future state of the climate is a complex, "wicked" problem that has been vastly oversimplified for political reasons. Given the unforecast flattening of temperatures for 18 years, there is no immediate crisis or urgency to the problem. In fact, I wouldn't even classify global warming as the world's most serious environmental problem. Worse are malaria in Africa and lack of drinkable water in many areas of the third world.

Thank you, Judy, for allowing my readers to see your thoughts on this important subject.

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