The Climate CIRCulator is brought to you by The Pacific Northwest Climate Impacts Research Consortium (CIRC) and The Oregon Climate Change Research Institute (OCCRI).

Colias butterflies in the Rocky Mountains are a thermally-sensitive species that face challenges due to climate change. National Park Service/Stacy Sink

Temperature Records Occur in Clusters

Studying the details of temperature fluctuations is essential to understanding how both individual organisms and whole ecoregions will adapt under climate change. But many studies could be missing an important nuance.

Most studies examining long-term changes in temperature tend to focus on trends in the mean, or average temperature, as well as on the highest of the high and the lowest of the low temperatures. In fact, a paper by CIRC researcher John Abatzoglou (University of Idaho), highlighted in the October CIRCulator, did just that for the Northwest, finding large increases in the coldest day of the year. Now a new study by Abatzoglou takes a different approach, examining the highest of the lows and the lowest of the highs.

The Abatzoglou study used data from 1,218 US Historical Climate Network stations from across the contiguous U.S. for the period 1920 to 2013. Nearly half of the stations had their highest maximum daily temperature (TMAX) in the 1930s during the Dust Bowl. During the same period, over a quarter of the stations also had their highest minimum daily temperature (TMIN).

Nearly 25 percent of highest TMAX records were set in a single month, July 1936, primarily at stations across the Great Plains and Midwestern U.S. (The records coincide with low soil moisture and broad-scale ridging.) Over 8 percent of the lowest TMAX records occurred in January 1985, coinciding with a cold-air outbreak that brought record low temperatures to the southeastern U.S. and widespread impacts to the Florida citrus crop. The period 2000–2013 was remarkable for its lack of lowest records (1 percent), and it had a moderate number of highest records (10 percent).

Employing 20 global climate models, Abatzoglou then examined the evolution of absolute temperature records through the mid-21st century under the high-end greenhouse gas scenario, Representative Concentration Pathway 8.5 (RCP 8.5). The simulations showed a continued increase in the prevalence of the highest TMAX and decreases in lowest TMIN through the mid-21st century.

Approximately half of all highest records for the period through 2049 occurred in the last decade of model simulations. By contrast, the occurrence of the lowest records declined in the 21st century with just 2 percent of such records occurring in the last decade of the period analyzed. In other words, high temperatures are expected to get higher and low temperatures are also expected to get higher. It’s still to be seen what this means for how organisms and ecosystems might adapt to these changes.

Abatzoglou, J.T., R. Barbero (2014) Observed and projected changes in absolute temperature records across the contiguous United States, Geophysical Research Letters, 41, 6501–6508. doi:10.1002/2014GL061441.

Idled land near Klamath Falls, Oregon. Department of Interior/Bureau of Reclamation

Weather and Climate
Finding New Ways to Link Weather Events to Climate Change

From Hurricane Sandy to this year’s drought in California and Oregon, extreme weather is now frequently associated with climate change in the popular imagination. For researchers, attributing these individual events to climate change has been tricky, though not impossible.

How can researchers know whether a given event is attributable to human-caused climate change? That is, did climate change increase the risk of an individual event occurring? A recent paper published in the Journal of Climate provides a rigorous mathematical framework for risks attributable to climate change. Researcher Gerrit Hansen and colleagues lay out a way to assess that risk using either observations or a climate model.

Alternative “attribution” studies use climate models to develop, in effect, “fingerprint” tests for individual events. When joined with historical weather station data, climate models can simulate historical conditions, similar to the methods reviewed in October’s CIRCulator article on coastal hazards. The advantage of this method is twofold: It increases the number of data points available for more robust statistical analyses, and it provides a way to “control” for the role of human influence by modeling a past with increasing levels of CO2 and one with stable CO2 levels.

OCCRI and CIRC researchers have used different attribution approaches for investigations into the role of CO2 in the following events:
  • The Texas drought of 2011
  • The central U.S. drought in 2012
  • Northern Hemispheric changes is spring snow cover
  • Pacific Northwest warming trends in the 20th century
Recently, the Bulletin of the American Meteorological Society’s annual special issue on extreme events contained three articles on the 2013-14 California-Oregon drought. None attempted a rigorous attribution, but two of the three compared features of the drought with comparable features in climate simulations. An article by Daniel Swain and colleagues noted that the ridge of atmospheric high pressure over the Pacific Ocean — the direct cause of the drought — was unprecedented in magnitude, and they estimated it had roughly a 400-year return period. They compared the frequency of such events in preindustrial climate model simulations with “climate change” simulations and found that such events became more likely. Hence, even without the direct effects of warming on moisture balance, the drought was made more likely by rising CO2.

NOTE: Hansen and colleagues used a “stochastic point process” or representation of randomly occurring events. They suggest that one can think of the baseline point process that would have occurred without climate change, and an additional point process of events that were in a sense “caused” or added by climate change.

Hansen G., M. Auffhammer, and A.R. Solow (2014) On the Attribution of a Single Event to Climate Change, J. Climate, 27, 8297–8301. doi:10.1175/JCLI-D-14-00399.1

Swain, D.L., M. Tsiang, M. Haugen, D. Singh, A. Charland, B. Rajaratnam and N.S. Diffenbaugh (2014) The extraordinary California Drought of 2013/2014: Character, Context and Role of Climate Change, [in "Explaining Extremes of 2013 from a Climate Perspective"].
Bulletin of the American Meteorological Society, 95(9), S3–S7. Full Report.

The figure above from the Synthesis Report links cumulative emissions, temperature change, impacts and near-term emissions changes. In sum: The more we emit, the more challenges emerge.

IPCC Synthesis Report
Connecting Causes and Impacts of Climate Change

Here’s what we know about climate change in a nutshell:
  • Human influence on the climate system is clear
  • Recent climate changes have had widespread impacts on human and natural systems
  • Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems.
These are the conclusions of the latest assessment report of the Intergovernmental Panel on Climate Change, summarized in the Synthesis Report released a few weeks ago. The Synthesis Report ties together themes from three earlier Working Group reports. The report is organized into four topics, summarized as follows:

Human influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history. Recent climate changes have had widespread impacts on human and natural systems.

Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems. Limiting climate change would require substantial and sustained reductions in greenhouse gas emissions, which, together with adaptation, can limit climate change risks.

Adaptation and mitigation are complementary strategies for reducing and managing the risks of climate change. Substantial emissions reductions over the next few decades can reduce climate risks in the 21st century and beyond, increase prospects for effective adaptation, reduce the costs and challenges of mitigation in the longer term, and contribute to climate-resilient pathways for sustainable development.

Many adaptation and mitigation options can help address climate change, but no single option is sufficient by itself. Effective implementation depends on policies and cooperation at all scales, and can be enhanced through integrated responses that link adaptation and mitigation with other societal objectives.

Intergovernmental Panel on Climate Change (2014) Climate Change 2014 Synthesis Report.

Lightning strikes the Warm Springs Indian reservation in 1999, photographed by a wildland fire engine crew member.

Wildfires and Climate Change
In a Warmer Northwest, There Are More Large Fires

Even under low-emissions greenhouse gas scenarios, the probability of very large wildfires increases by at least 30 percent by 2100 in the West, according to a new study led by Natasha Stavros of NASA’s Jet Propulsion Laboratory. The Rocky Mountain region showed the largest increase in probability, followed by the Pacific Northwest.

The Stavros study, coauthored by CIRC researcher John Abatzoglou, lends important new data to several previous studies linking climate change and variability to wildfire extent in the western United States. Stavros and colleagues used a detailed statistical approach to link very large wildfires (more than 50,000 acres) to weekly variability in weather and weather-derived variables such as temperature, fuel moisture and energy release across eight regions in the western U.S. defined by fire-management agencies.

In most regions there’s a noticeable difference between low-emissions greenhouse-gas scenarios (RCP 4.5) and high-emissions scenarios (RCP 8.5) in the probability of large fire occurrence linked to weather conditions on weekly timescales. The study also looks at how those conditions will change in the future. Unlike studies using only fire-danger indices or changes in annual area burned, the current study derives relationships between climate and the likelihood of large fires directly.

Their approach worked well to simulate past fire occurrence. The authors compared observed past fire occurrence in each of the eight regions with simulated fire occurrence using weather output for the past from climate models. For seven of the eight regions (the western Great Basin being the exception), the modeled and observed probabilities were statistically indistinguishable.
Stavros, E.N., J.T. Abatzoglou, D. McKenzie, and N.K. Larkin (2014) Regional projections of the likelihood of very large wildland fires under a changing climate in the contiguous Western United States, Climatic Change 126:455–468. doi:10.1007/s10584-014-1229-6

Cows grazing in Humbolt County, Calfornia. Joel Rogers

Rangelands and Climate Change
Warmer Temperatures Could Boost
Plant Growth on Rangelands

East of the Cascade mountains, hundreds of thousands of cattle graze the vast rangelands. This economically vital agricultural region of the Pacific Northwest could become more productive as a result of climate change, a new study finds. Climate models show that rangelands in the interior West — including eastern Oregon and Washington as well as southern Idaho — would experience the greatest increases in productivity, according to the study by the Rocky Mountain Research Station of the U.S. Forest Service.

Matthew Reeves and colleagues compared the conversion of CO2 into plant growth under four climate scenarios as measured by “net primary production” (NPP) of rangelands across the continental United States. Specifically, the study estimated that production would initially decrease in the first half of the 21st century but rebound and increase by more than 25 percent from historical levels by century’s end.

These gains in net primary production seem to result from projected increases in “CO2 fertilization” — that is, enhanced growth resulting from higher concentrations of carbon dioxide in the atmosphere. Lab studies have attributed this extra growth to greater efficiency of water use. In other areas of the United States, precipitation is the most likely driver of increased growth, particularly in the prairies of Montana and the Dakotas to the north; Nebraska, Kansas and Oklahoma to the south; and Iowa, Missouri and Arkansas to the east. In the Southwest rangelands, on the other hand, increasing temperature is likely to drive losses of production.

What this may mean for rangeland management, particularly for grazing, is unclear. The paper cites other studies suggesting that increased NPP could raise carrying capacity for grazing in areas of Australia by more than 40 percent, while other studies contend that overall carrying capacity could decline even with increased NPP because of other limiting factors such as plant nitrogen production. Also note that the study does not consider other factors impacting rangelands, including how pests (such as grasshoppers) or invasive species (such as cheat grass) will respond to future climate conditions.

Reeves, M.C., A.L. Moreno, K.E. Bagne (2014) Estimating climate change effects on net primary production of rangelands in the United States, Climatic Change, 126:429. doi:10.1007/s10584-014-1235-8e

Fig. 4 Mean slope of linear regression of NPP trend for U.S. rangelands to 2100 (a). Standard deviation of the mean slope of linear regression of NPP trend to 2100 for A1B, A2, and B2 emissions scenarios (b).

The Climate CIRCulator is brought to you by The Pacific Northwest Climate Impacts Research Consortium (CIRC). CIRC delivers science, information, and tools to decision makers responsible for the management of  resources and services in a changing climate. Our team consists of experts from Oregon State University, the University of Oregon, the University of Idaho, and the University of Washington. CIRC is funded by the National Oceanic and Atmospheric Administration (NOAA) and housed in the Oregon Climate Change Research Institute (OCCRI) at Oregon State University. The OCCRI brochure can be downloaded  here.

The Climate CIRCulator, November/December 2014, Issue 10.
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