To what degree is the modest warming over the northwest U.S. during the last century caused by (1) natural variations or (2) by increases in greenhouse gases emitted by mankind?
This is a critical question with huge implications for public policy, climate adaptation, and scientific understanding of local climate change.
The two viewpoints
On one hand, there are some scientists saying that nearly all of the warming is due to mankind's emission of greenhouse gases. For example, here is a quote (made to KUOW) by Dr. Phil Mote, a scientist at Oregon State and Oregon State climatologist:
“As far as the 1.3 degree warming over the last hundred years or so," Mote said, "that’s all because of human activity.”
He is claiming that all of the warming of the last 100 years is due to anthropogenic (human-related) forcing. And Dr. Mote goes even farther than that, suggesting that the local warming has accelerated
"what is particularly significant is that the rate of warming is increasing"
Support for his claims is found in a paper he published in the Journal of Climate last April with John Abatzoglou (the lead author, University of Idaho) and David Rupp (Oregon State):
On the other hand, a recent paper by Drs. Jim Johnstone and Nate Mantua in the prestigious journal Preceedings of the National Academy of Sciences (PNAS) came to the OPPOSITE conclusion:
"these changes are not likely related to historical anthropogenic and natural radiative forcing"
As Dr. Mantua said to the Seattle Times:
"We do not see a human hand in the warming of the West Coast,”
In their paper, Dr. Johnstone and Mantua demonstrate that natural variability explains virtually all of the temperature evolution of the past century over Northwest U.S.
So we have two groups of scientists, both with Ph.Ds and having published results in well-known academic journals, coming to the opposite conclusions about the origin of the roughly 1.3 degree F warming that has occurred over the Northwest in the last century.
How can two groups of scientists looking at the same basic data come to radically different conclusions?
Both can not be correct.
As explained below, I believe the Johnstone/Mantua paper makes a far better case: human-caused warming over the Northwest has been minimal, with natural variability dominating.
Both teams of atmospheric scientists agree that the earth and the Pacific Northwest will warm due to increases in greenhouse gases, what they disagree upon is the impact of greenhouse gases in the past compared to natural variability.
The warming of the Pacific Northwest
One things nearly everyone agrees upon is that the Pacific Northwest has warmed over the past century, but NOT in a continuous way. Here are two plots of temperature change (from a base period) over the Northwest, one from the Abatzzoglou, Rupp, and Mote paper (upper figure) and the other from the summary report of the UW Climate Impacts Group (bottom). The top figure has the temp variations from three different sources as well as lines that attempt to smooth out the yearly variations. The second figure has a single trend line (which really doesn't make sense since the variation is so complex)
These plots can be summarized as follows:
- Since 1900, the temperatures have warmed by about about 1.3 F
- The temperature has NOT warmed in a steady way.
- The largest warming was concentrated in two periods: 1915-1940 and 1975 to 1985
- Temperature has hardly changed from roughly 1985, the last 30 years.
Natural versus human-forced variability
So what is forcing the temperature changes shown above?
Let's begin by human-connected temperature changes forced by increasingly greenhouse gases (such as CO2). The Intergovernmental Program for Climate Change (IPCC) has published a series of authoritative reports, including an estimate of the global impacts on radiation in the atmosphere due to mankind's influence on greenhouse gases. Here is a plot of their estimates over time; pay particularly attention to their estimates of the influence of Long Lived Greenhouse Gases (LLGHG, red line))
Clearly, the temperature change in the Northwest looks very different than this, so more must be going on!
For example, because of the non-uniform nature of the earth's surface and weather systems, some places would warm up more or less than others. We can get a handle on that by looking at the average of many climate simulations for the upcoming century (as shown in the figure below, which shows the impact of greenhouse gas increases on surface temperatures by the year 2100). The Arctic warms up the most, the continents warm more than the oceans, and the eastern oceans warm up less than the western oceans. The Northwest is downstream of the eastern Pacific and thus would warm less than most places (the simulations below did not have the resolution to describe our cooling by the Pacific, higher-resolution simulations show this).
So what should you conclude? The radiative effects of greenhouse gases were relatively small before roughly 1965 and that our area should experience less warming than most places.
There is nothing controversial in this statement. Thus, one would certainly not expect the warming in the early part of the century over our regon to be forced by mankind's greenhouse gas emissions.
But although the Northwest should not have been highly impacted by global warming from greenhouse gases, we ARE highly impacted by natural variability, changes in atmospheric circulations that occur due to complex interactions within the atmosphere and with the oceans and land surface.
One example many of you know: El Nino and La Nina, in which sloshing water in the tropical Pacific causes an oscillation of warming and cooling of equatorial waters over a period of 4-7 years. El Nino/La Nina, often called ENSO (El Nino Southern Oscillation), has weather impacts all over the world.
But there is another type of natural variability that has a huge impact on the weather/climate of the Pacific basin: The Pacific Decadal Oscillation. This climate feature, discussed in depth for the first time by Dr. Nate Mantua (yes the same Mantua of the paper noted above), has a period of around 50 years, oscillating between warm and cold cycles (see figure). It was in a cool cycle between roughly 1950 and 1977, then a warm cycle until around 2005, and more recently looks to be in a cooler cycle.
If you compare the variations of the PDO with the Northwest temperature traces shown above, it is very obvious that the variations of NW temps seems to closely follow the PDO changes, suggesting our temperatures are highly controlled by this mode of natural variability.
Thus, when some local scientists say that the temperature changes experienced here in the Northwest are mainly due to greenhouse gas emissions they are certainly incorrect. We live in an area where the greenhouse gas signal is small and where natural variability (as forced by the PDO and ENSO) are quite large (and there are other modes of natural variability I have not even discussed).
As noted earlier, the media has been full of reports and stories about this controversy of natural versus greenhouse gas forcing here in the Northwest. There have been literally hundreds of stories in national/international media outlets and the Seattle Times even wrote an editorial about the subject.
Many of the media reports were stimulated by the Johnstone/Mantua paper in PNAS. This paper presented stunning results. They showed that one could explain the sea surface temperature pattern off the West Coast if one knows the pressure/wind field. Amazingly, they could produce a nearly perfect evolution of the temperatures over the past 100 years using pressures/winds (a.k.a., the circulation).
Here is an example of their results from this paper. SLP1 is their pressure/circulation index, SSTarc is the sea surface temperatures off the West Coast, and SATarc are air temperatures over land along the West Coast. Folks, this is amazing correspondence.
You might ask, could the changes in the winds/pressure that explain the temperature changes be explained by greenhouse gas increases? At this point in time, there is no theoretical or observational evidence to suggest so, and tree ring data suggests long term circulation changes have occurred for thousands of years in the past. Furthermore, when Johnstone/Mantua examined the pressure/circulations changes produced by global climate models over the next century, they did NOT find any suggestions of changes in natural variability resulting from greenhouse gas increases.
So the bottom line of the Johnstone/Mantua paper is that natural variability has dominated temperature changes during the past century over the Northwest. I believe their arguments are very strong.
In contrast, there is the paper of Abatzoglou, Rupp, and Mote (2014) that suggests greenhouse gases are the dominant source of warming. Specifically, they stated that anthropogenic forcing was "the leading contributor to long term warming." In their paper, they used a technique called multiple linear regression to determine the forcing of temperature by several forcing mechanism (natural variability, volcanoes, solar variability, and human greenhouse gas forcing). Their results suggested that anthropogenic contributions (greenhouse gases) were dominant. But unfortunately, their approach has some critical problems that make their conclusions insupportable. Problems so severe I am surprised they got through the review process.
The biggest deficiency--and this is going to get technical-- is that they detrended (over time) their index of natural variability (they detrended the 500-hPa height anomalies they used). In doing so, they removed the ability of natural variability to explain the long-term temperature trends. In other words, they threw away EXACTLY what Johnstone/Manuta showed to be important.
And the Abatzoglou paper had other problems, like using a poor index of natural variability (they did not use the PDO or anything like it).
I know this has been a long blog, but the issues are very important. What should you come away with?
- Global warming is not globally uniform, some places will warm more or less quickly. The Northwest is a regional where warming will be slow due to our proximity of the eastern Pacific.
- Natural variability in climate can be quite large and in some locations it is currently far larger than the contribution of mankind from greenhouse gas emissions and other impacts.
- For the West Coast, natural variability has been far larger than greenhouse gas warming.
As the century progresses, the human-caused global warming signal will increase while natural variability should remain about the same. Thus,the lack of human-caused warming on the West Coast does not imply that anthropogenically forced global warming is unimportant. It is a serious issue mankind must deal with.
This situation also shows you some of the strengths and weaknesses of the scientific process. Some papers get published that have problematic results. Some scientists say things that are not supported by rigorous research. But the system has a way of righting itself and self correcting over a period of time. The Johnstone/Mantua paper has made a substantial contribution to recentering the discussions regarding Northwest climate change.