Is PNW warming human caused or not? Does it matter to agriculture?

By Chad Kruger

Reprinted from: WSU CSANR Perspectives on Sustainability

The observed temperature records of the US Pacific Northwest show a small, but statistically significant amount of warming of just over 1 degree F since the year 1900. A paper published in March of this year by Abatzaglou, Rupp and Mote (2014) used a multiple linear regression model to “tease out” the contributions of different influences on climate and “to apportion trends to internal climate variability, solar variability, volcanic aerosols, and anthropogenic forcing [a.k.a. human greenhouse gas emissions]”. Unsurprisingly, the finding of this study was as expected:

Anthropogenic forcing was a significant predictor of, and the leading contributor to, long-term warming; natural factors alone fail to explain the observed warming.

And, then, this little bombshell was published this week …

Jim Johnstone and Nate Mantua NOAA just published a paper in the Proceedings of the National Academy of Sciences (PNAS) using a single linear regression model to correlate sea level pressure in the NE Pacific ocean with sea surface temperatures (SST) and surface air temperatures (SAT) in the coastal PNW. Their finding?

NE Pacific circulation changes are estimated to account for more than 80% of the 19002012 linear warming in coastal NE Pacific SST and US Pacific northwest (Washington, Oregon, and northern California) SAT. An ensemble of climate model simulations run under the same historical radiative forcings fails to reproduce the observed regional circulation trends. These results suggest that natural internally generated changes in atmospheric circulation were the primary cause of coastal NE Pacific warming from 1900 to 2012….

Got that? In layman’s terms, they conclude that the observed 1 degree F of warming observed over the last century in the US Pacific Northwest was due to changes in wind patterns, and that those changes in wind patterns derived from natural variability not greenhouse gas emissions from human sources.

I usually don’t wade into the “controversy” regarding natural vs. human-caused warming because a) I’m not an atmospheric scientist and therefore don’t have sufficient training to interpret methodologies and b) the causal linkage between elevated CO2 and climate doesn’t really matter when it comes to evaluating the impact climate change on agriculture. We can isolate and test the sensitivity of the effects of warming, changes in precipitation and elevated CO2 on crops, livestock and hydrology regardless of the connectivity between these variables and the cause of the change (in fact, we do just that). However, because this new study raises questions regarding how climate models forecast those future variables in our region (and we do use those forecasts), I felt it was important to note that we are cognizant of this study and anxiously awaiting resolution.

Abatzaglou, Rupp and Mote have already raised concerns about the selection of the dataset used by Johnstone and Mantua for the PNAS paper. Abatzaglou, Rupp and Mote point to the large disagreement pre-1950 between available datasets on sea level pressure (see first figure here) to indicate that the use of one or more of several different data sets with the same methodology presented in the PNAS paper may actually result in a finding more in line with their own findings (Johnstone and Mantua used only the red NCAR dataset to perform their analysis).

This new “controversy” is likely to take some time to resolve itself as more studies and analyses are conducted to evaluate the inconsistency that Johnstone and Mantua have presented. That’s what should happen in science, but it doesn’t mean that we should readily discount everything we think we know. We already know that the global climate models (GCMs) don’t capture all of the critical phenomena that drive regional climate – and that GCMs might underestimate important concerns. The question this PNAS study raises is important to resolve on many levels and may be useful for improving regional climate forecasting, but even Johnstone and Manuta indicate that their finding does not change the current interpretation of expected future impacts of anthropogenic warming in the region.

Perhaps there will need to be some recalibration of existing climate forecasts, but considering the fact that published forecasts we use already include an extremely wide spread of possible warming (ranging from ~1–9 degrees over this century) there is plenty of room for considering a 1 degree margin of error as it relates to crops, livestock and water supply. As I’ve discussed before, a few degrees of warming is likely to be noise relative to inter-annual climate variability for most agricultural systems in the PNW, but more warming than that begins to be cause for concern. At this point, this paper simply reinforces that idea.

The fact is that global greenhouse gas emissions are currently increasing faster than the highest Intergovernmental Panel on Climate Change (IPCC) projected emissions scenario – and if the currently assumed causal link between emissions and temperature is even half correct, we’ll still have plenty of change to manage in the future, which I’ll address in a future post.


Abatzoglou, J. T., D.E. Rupp, & P.W. Mote (2014). Seasonal climate variability and change in the Pacific Northwest of the United States, Journal of Climate, 27, 2125-2142,Online Access

Johnstone, J.A. and N.J. Mantua (2014). Atmospheric controls on northeast Pacific temperature variability and change, 1900-2012. PNAS. Online Access


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