Late last year the Intergovernmental Panel on Climate Change (IPCC) released its Fifth Assessment Report on climate change and earlier this month, the U.S. Global Change Research Program released its third national assessment of Climate Change Impacts in the United States. Both reports highlight the increased certainty with which climate change has been occurring, its anthropogenic (resulting from the influence of human beings on nature) cause, and its expected continuation.
Both reports announced this increased certainty from a global temperature perspective. Each of the past three decades has been warmer than the previous. And the most recent one has been the warmest. Since the mid-1800s, the combined land-ocean temperature has increased by ~0.85°C. There has also been an observed increase in the number of warm nights, heat waves, and many other temperature-related phenomena.
Because of the large-scale nature of the change, and because the theoretical link between an increase in CO2 and warming is well understood, and because global climate models are able to reconstruct most of the observed change as well as project future warming, there is high degree of confidence that most of these temperature-related characteristics of climate change will continue.
The degree of certainty regarding the impact on non-temperature-related weather phenomena decreases. As the scale of the phenomenon and the region in question gets smaller, the confidence about how climate change will impact weather diminishes.
For example, while it is likely that there have been statistically significant increases in the number of heavy precipitation events in some regions since 1950, regional and sub-regional trends do not always tell a consistent story. Parts of North America have shown increases in heavy precipitation events. But, on virtually every other continent, there is low-to-medium confidence of any historical trends. Future projections of increases in heavy precipitation events range from mostly low-to-medium, to high in some high mid-latitude regions like Canada and Alaska. The expected reason is a northward shift in extratropical cyclone (ETC) activity.
In fact, for ETCs-which are large storms that typically affect mid latitudes in winter with heavy winds, snow, and rain-there is medium confidence that the storm tracks in both hemispheres have shifted poleward, but low confidence that the change is forced by human activity. And, while ETCs are resolvable by general circulation models (GCMs), the processes that lead to their intensification, like the release of latent heat of condensation, can be very small scale and difficult to capture.
That is why, even with the expected reduction in the pole to equator temperature difference that typically energizes such storms, it is not clear whether latent heating will compensate to yield (more) stronger storms. For now, the IPCC expects changes in regional ETC activity, but has low-to-medium confidence in the details of those changes.
For tropical cyclones (TCs), which are even smaller than ETCs, there is low confidence that any long-term (i.e., 40 years or more) increase in activity has even occurred. The incomplete understanding, coupled with the fact that GCMs still cannot explicitly resolve TCs, makes it difficult to project how climate change will impact such activity by the end of this century.
There is more to hurricanes than just warm ocean water. Vertical wind shear, humidity, tropopause temperature, and disturbances can all act as triggering mechanisms. Understanding and accurately modeling how all of these ingredients will evolve in the next 80-100 years is still a challenge for GCMs. The IPCC updated their 2007 prediction of a likely increase in global TC activity to project an increase in cyclone intensity, but not frequency. And that is just for TC formation. Nothing about landfall.
So what about severe thunderstorms, hail, and tornadoes? These very small scale weather phenomena are difficult to forecast even with today's high-resolution weather models. The hope for understanding how climate change will impact them is to have a better grasp of the large-scale ingredients necessary for their development, which is currently not an easy task.
We know that a dozen or more ingredients like convective available potential energy and vertical wind shear are important for severe storm development. But unfortunately even knowing all those variables cannot give us a clear answer. Why not? Well for one thing, these parameters measure the potential for such activity but not whether such activity will actually occur. Other features like surface weather fronts, sea breezes, and other micro-scale forces that can kick a moist air parcel from the surface upward to spawn a supercell thunderstorm are still difficult to forecast.
All this talk of medium and low confidence might suggest that little advancement of our understanding of climate change and weather phenomena has occurred in the last 7 to 8 years, but that statement couldn't be farther from the truth.
In reality, we as a scientific community have learned quite a bit about individual weather phenomena as it relates to climate. We have learned that projecting the impacts of climate change to extreme weather events is very, very, complicated. And we have learned that some of the earlier projections about changes in weather phenomena were based on an understanding that was incomplete and too simplistic.
Decreased certainty for now does not mean less understanding. It just means there is more uncertainty-for now. That may be one step backward. But better understanding of physical processes, more accurate parameterization of those processes in GCMs, and faster computers able to run them at higher resolution will be three steps forward. These steps will likely advance our understanding and decrease our uncertainty by the time the next round of assessments rolls out.