The University of Arizona

Same Old Story?

October 4, 2013
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Shoot an arrow at the center of climate change and it may well end up in the Southwest. Last week, the Intergovernmental Panel on Climate Change (IPCC) released the first portion of the Fifth Assessment Report (AR5), known as the Physical Science Basis, which considers the current state of climate change across the globe. I was privileged to attend a panel discussion of the report comprised of four climate experts from the University of Arizona—Julie Cole, Russ Monson, Joellen Russell, and Mike Crimmins. Towards the end, Cole said something that stuck with me: “Here in Arizona, we are in the bull’s-eye of climate change.” We’ve heard repeatedly that the Southwest is warming and that we’ll get drier, but aren’t other regions getting warmer and drier also? What makes us the perfect target for climate change?

First, according to Cole, “we are warming faster than almost anywhere else.” According to the newest temperature projections described in AR5, under the highest emissions scenario—a future with very little to no mitigation leading to very high greenhouse gas emissions—the average global temperature will be 6.7°F warmer by 2100. But in the Southwest, we’re looking at an average that will be 10.8°F warmer (figure 1; note it uses a Celsius temperature scale). Granted, this is in a future in which we have done very little to combat climate change, which I would like to think won’t happen. Nevertheless, the difference in temperatures shows the significance of warming in the Southwest, and it’s not good news.

Figure 1: Annual mean surface temperature change (degrees Celsius) in 2081-2100 relative to 1986-2005. Results shown are for two emissions scenarios, known as representative concentration pathways (RCPs). RCP2.6 is a scenario where mitigation leads to very low CO2 concentrations (about 20 ppm higher than current concentrations). RCP8.5 is the highest emissions scenario with concentrations reaching 936 ppm (or about 535 ppm higher than today) by 2100. Reference: Figure SPM.8 from the Summary for Policy Makers of Working Group 1 Report (the Physical Science Basis) to the AR5.

Furthermore, Russell noted that in recent years, “the oceans are taking up over 93% of the warming,” meaning the slowdown in average global temperature rise over the past 15 years or so is because the oceans are taking up more heat, giving us a tiny reprieve (a recent article in the Arizona Daily Star talks more about this). However, since the climate exhibits natural variability and shifts in patterns, “we expect to see more heat upon us in the future.”

Returning to why the Southwest is the perfect target for climate change, Cole explained that this intense warming will make us much drier, regardless of the predicted changes in precipitation. The new report reiterates the oft-made prediction that wetter regions will get wetter and drier regions will get drier, meaning precipitation in the Southwest is likely to decrease. However, although projections for future precipitation in our region are not nearly as robust as those for temperature, we know that as the air warms, the soils will get drier. This is true across the globe, but especially in already-dry areas like ours. Because of this, “heat is one of the most important factors when predicting future drought in the Southwest,” according to Cole.

Cole also discussed potential changes to the monsoon, including a delayed onset and more intense precipitation, although not necessarily more total precipitation. One of our most unpleasant times of year is the hot, dry season leading up to the monsoon; in the future, this miserable period will likely be longer. But the monsoon storms will be more intense, which should be a good thing, right? Well if we have no change in overall precipitation but the individual storms are heavier, then there will be longer times between storms and increased risk of flooding when they hit.

Cole added one more reason why the Southwest is in the bull’s-eye of climate change impacts: river runoff, as we have heard time and time again, is projected to decrease, largely due to warmer temperatures melting snowpack and increasing evaporation. But the nail in the coffin is our rapid population growth, which will not only increase demand on resources such as water and electricity, but will mean greater numbers are at risk for flooding and other climate extremes, such as heat, leading to greater economic and public health costs.

Much of this may seem like the same old story; does this latest report actually say anything new? We know temperatures are rising and will continue to rise, we know snowpacks are melting, we know sea level is rising, and we know humans are causing at least some of these changes. The report states, “warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased. […]. Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system” (SPM-3, SPM-10).

A lot of the report is the “same old story”. But the panelists remarked that, in a way, this is a good thing. Even with better climate models and more research, we are seeing the same picture, giving us greater confidence in our predictions. We are able to better constrain what we already know by reducing the amount of uncertainty surrounding our knowledge. And we are learning new things, especially with respect to the ocean, where scientists are constantly collecting new data and learning how heat distributes through all depths. To quote Cole, “scientists do agree on climate change. Now we need to try to manage the risks associated with it.”


* The AR5 consists of 4 separate reports: 1)The Physical Science Basis was released last week, the reports on 2) impacts, adaptation, and vulnerability, and 3) mitigation will be released next spring, and 4) the synthesis report will come out in fall 2014.