New Paper: Unprecedented 21st-century drought risk in the American Southwest and Central Plains

GUEST POST by Bob Tisdale

That’s the title of a new paper by Cook et al. that’s been making the rounds in the mainstream media.

The paper is available from GISS here. The abstract reads:

In the Southwest and Central Plains of Western North America, climate change is expected to increase drought severity in the coming decades. These regions nevertheless experienced extended Medieval-era droughts that were more persistent than any historical event, providing crucial targets in the paleoclimate record for benchmarking the severity of future drought risks. We use an empirical drought reconstruction and three soil moisture metrics from 17 state-of-the-art general circulation models to show that these models project significantly drier conditions in the later half of the 21st century compared to the 20th century and earlier paleoclimatic intervals. This desiccation is consistent across most of the models and moisture balance variables, indicating a coherent and robust drying response to warming despite the diversity of models and metrics analyzed. Notably, future drought risk will likely exceed even the driest centuries of the Medieval Climate Anomaly (1100–1300 CE) in both moderate (RCP 4.5) and high (RCP 8.5) future emissions scenarios, leading to unprecedented drought conditions during the last millennium.

The paper has two strikes against it right from the get-go:  paleoclimatological data and climate models.

A COUPLE OF QUICK SPOT CHECKS

The Cook et al. (2015) paper states, where PDSI stands for Palmer Drought Severity Index:

PDSI is easily calculated from GCMs using variables from the atmosphere portion of the model (for example, precipitation, temperature, and humidity) and can be compared directly to observations.

So let’s take a quick look a couple of worst-case examples of how poorly the models simulated temperature and precipitation in the regions selected by Cook et al during the satellite era, the past 35 years.

They selected a group of 17 models from the CMIP5 archives, using RCP4.5 (moderate emissions scenario) and CP8.5 (“business as usual” scenario).  As a spot check, the following two model-data comparisons use the average of all of the models in the CMIP5 archive, with the historic forcings from 1979 to 2005 and the RCP8.5 scenario afterward.  If you’d like to redo the following graphs with only the models used by Cook et al., you’re more than welcome to do so. And also show us the outputs of the models that Cook et al. didn’t use.

Cook et al. also identified the coordinates of the regions they included in their study:

All statistics were based on regional PDSI averages over the Central Plains (105°W–92°W, 32°N–46°N) and the Southwest (125°W–105°W, 32°N–41°N).

And their paper included the boreal summer months of June-July-August.

For the data in the following comparisons, we’re presenting GISS Land-Ocean Temperature Index data, and CAMS-OPI precipitation data, which is a merger of rain gauge and satellite-based precipitation data.  The data and the climate model outputs are available from the KNMI Climate Explorer.

Again, we’re showing the worst case model-data comparisons.

For the Southwest United States region, the climate models are showing almost twice the observed June-July-August precipitation from 1979 to 2014. See Figure 1.

Figure 1

And in the Central Plains region of the United States, the models more than double the observed warming rate.

Figure 2

CLOSING

My Figure 3 is Figure 1 from Cook et al. (2015).  Nice hockey stick.

Figure 3

Maybe at some time in the future, probably not in my lifetime, the climate science community will come to realize that model outputs showing “unprecedented” future values are indications the models are fatally flawed.