An unsettled climate change debate
In a press conference last week, U.N. Secretary-General Ban-Ki Moon stated: "Action on climate change is urgent. The more we delay, the more we will pay in lives and in money." The recently appointed U.N. Messenger of Peace Leonardo DiCaprio stated "The debate is over. Climate change is happening now."
These statements reflect a misunderstanding of the state of climate science and the extent to which we can blame adverse consequences such as extreme weather events on human caused climate change. The climate has always changed and will continue to change.
Humans are adding carbon dioxide to the atmosphere, and carbon dioxide and other greenhouse gases have a warming effect on the climate. However, there is enduring uncertainty beyond these basic issues, and the most consequential aspects of climate science are the subject of vigorous scientific debate: whether the warming since 1950 has been dominated by human causes, and how the climate will evolve in the 21st century due to both natural and human causes.
At the heart of the recent scientific debate on climate change is the "pause" or "hiatus" in global warming — the period since 1998 during which global average surface temperatures have not increased. This observed warming hiatus contrasts with the expectation from the 2007 IPCC Fourth Assessment Report that warming would proceed at a rate of 0.2oC/per decade in the early decades of the 21st century. The warming hiatus raises serious questions as to whether the climate model projections of 21st century have much utility for decision making, given uncertainties in climate sensitivity to carbon dioxide, future volcanic eruptions and solar activity, and the multidecadal and century scale oscillations in ocean circulation patterns. If the recent warming hiatus is caused by natural variability, then this raises the question as to what extent the warming between 1975 and 2000 can also be explained by natural climate variability.
A key argument in favor of emission reductions is concern over the accelerating cost of weather disasters. The accelerating cost is associated with increasing population and wealth in vulnerable regions, and not with any increase in extreme weather events, let alone an increase that can be attributed to human caused climate change. The IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation and the 5th Assessment Report find little evidence that supports an increase in most extreme weather events that can be attributed to humans. There seems to be a collective "weather amnesia," where the more extreme weather of the 1930s and 1950s seems to have been forgotten.
Climate science is no more "settled" than anthropogenic global warming is a "hoax." I am concerned that the climate change problem and its solution have been vastly oversimplified. Deep uncertainty beyond the basics is endemic to the climate change problem, which is arguably characterized 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.
Nevertheless, the premise of dangerous anthropogenic climate change is the foundation for a far-reaching plan to reduce greenhouse gas emissions and reduce vulnerability to extreme weather events. Elements of this plan may be argued as important for associated energy policy reasons, economics, and/or public health and safety. However, claiming an overwhelming scientific justification for the plan based upon anthropogenic global warming does a disservice both to climate science and to the policy process. Science doesn't dictate to society what choices to make, but science can assess which policies won't work and can provide information about the appropriate decision analytic framework for decision making given the level and types of uncertainty in the science.
Can we make good decisions under conditions of deep uncertainty about climate change? Uncertainty in itself is not an excuse for inaction. Research to develop low-emission energy technologies and energy-efficiency measures are examples of "robust" policies that have little downside and ancillary benefits beyond reducing greenhouse gas emissions. However, attempts to modify the climate through reducing CO2 emissions may turn out to be futile. The stagnation in greenhouse warming observed over the past 16 years demonstrates that CO2 is not a control knob on climate variability on decadal time scales. Even if CO2 mitigation strategies are successful and climate model projections are correct, an impact on the climate would not be expected until the latter part of the 21st century.
It's time to recognize the complexity and wicked nature of the climate problem, so that we can have a more meaningful dialogue on how to address the complex challenges of climate variability and change.
Judith Curry is professor of Earth and atmospheric sciences at the Georgia Institute of Technology and president of Climate Forecast Applications Network.