Origins of the 2012 Central Great Plains Drought - Frequently Asked Questions
This FAQ covers questions asked about the report "Interpretation of the Origins of the 2012 Central Great Plains Drought" which is an assessment report of the NOAA Drought Task Force Narrative Team.
1. Why was this assessment done?
The assessment was done to explain causes for the extreme 2012 Central Great Plains drought with a particular goal to better understand the event's predictability. The assessment was therefore also done to explain the nature of the prediction challenge, and to provide reasons for the lack of early warning for the 2012 drought. This assessment of causes, placed in the context of forecast diagnosis, had as an overarching purpose to identify pathways for better predictions of future drought events.
2. Who was involved in this study?
The Report to assess the causes and predictability of the 2012 drought was prepared by the Narrative Team of NOAA's Drought Task Force in partnership with the National Integrated Drought Information System (NIDIS). The Drought Task Force is organized by the Climate Program Office Modeling, Analysis, Prediction and Projections Program (MAPP) and consists of over thirty leading drought and climate scientists from multiple academic and federal institutions funded by the program. The Drought Task Force involves scientists from NOAA's research laboratories and centers, NASA, U.S. Department of Agriculture, NCAR and several universities - the University of California-Irvine, the University of South Carolina, the University of Maryland, Michigan State University, the University of Colorado, and Columbia University.
3. What caused the 2012 Central Great Plains Drought?
The Report concluded that the Central Great Plains Drought during May-August 2012 resulted mostly from natural weather variations. The Report found:
- Moist Gulf of Mexico air failed to stream northward in late spring as cyclone and frontal activity were shunted unusually northward.
- Summertime thunderstorms were infrequent and when they did occur produced little rainfall.
- Neither ocean states nor human-induced climate change - factors that can provide long-lead predictability and an early warning capability - appeared to play significant roles in causing severe rainfall deficits that were of record intensity over the Central Great Plains and extended over much of the Nation's midsection in summer 2012.
4. Is there contradiction between the Report's finding that the 2012 drought was mostly unrelated to human-induced climate change, and the overwhelming body of scientific evidence that global warming is real?
No, scientific findings of natural causes for local weather or climate extremes such as the 2012 drought do not contradict the scientific findings affirming human-induced (anthropogenic) global warming. As stated in the 2012 IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX):
"Many weather and climate extremes are the result of natural climate variability (including phenomena such as El Nino), and natural decadal or multi-decadal variations in the climate provide the backdrop for anthropogenic climate changes. Even if there were no anthropogenic changes in climate, a wide variety of natural weather and climate extremes would still occur."
The SREX (2012) report also states:
"Projected changes in climate extremes under different emissions scenarios generally do not strongly diverge in the coming two to three decades, but these signals are relatively small compared to natural climate variability over this time frame. Even the sign of projected changes in some climate extremes over this time frame is uncertain."
Global warming refers to the increase in the average surface temperature of Earth's atmosphere and oceans that has occurred since the late 19th century. This global warming is projected to continue through the 21st century due to human-induced changes in the atmosphere's trace gas composition, principally from increases in CO2. In contrast to temperatures, the effects of the changing atmospheric composition on precipitation are far less systematic and well-understood, especially over a small region like the Central Great Plains. Most scientific results to date suggest that in upcoming decades natural weather patterns are, and will remain, the dominant factors in driving regional precipitation variations occurring over months to seasons, as in the 2012 drought, even as human-emitted greenhouse gases are likely to produce additional global warming through the 21st century.
5. How difficult is it to assess whether global warming influences short-term and local weather patterns?
This is one of the most difficult challenges in climate science. In general, the effects of global warming relative to natural climate variability become much more difficult to detect and attribute when going from large space and long time scales (e.g., century long global trends in surface temperature) to the short time periods and small spatial scales that characterize most extreme events, which are more typical of weather variability. The science framing for explaining causes for extreme events is two fold:
1. Could an extreme event have been anticipated to happen over a given region, within a particular time window, and with a particular intensity?
This is a forecast problem, that has a primary aim to provide early warning to the public and decision makers. Part of the Report examined the predicability of the 2012 drought. The Report found that the 2012 drought was not anticipated by operational prediction systems obtained from different modeling centers around the globe. The Report examined why it wasn't anticipated, in part to determine what steps might be taken to better anticipate and provide early warning of such events in the future.
2. How does global warming affect the likelihood of the event?
Let's take a hypothetical situation in which there was reason to believe that climate change had increased the odds for an extreme event to occur. One relatively simple example would be the odds for a record-setting hot summer. The odds due to long-term warming would have been almost equally elevated in the prior year (2011) as in the subsequent year (2013) or for that matter for several years either before or after the event actually occurred. The odds would also have increased about the same amount for other portions of the contiguous U.S, because of the relative uniformity of the warming signal.
Thus, global warming alone does not provide specific information required for a forecast or early warning system, but generally does contribute toward higher temperatures, making record-warm values more probable. At the same time, it is important to recognize that the boost in temperatures is likely to be only a small fraction of the total intensity of a heat wave; for example, at this point in time, it might add 1º C to a 10º C above normal heat wave, with the remainder of the event's extreme magnitude (9º C) to be explained by other causes. For specific heat waves, weather variability is generally of primary importance for determining the event's overall magnitude, location, and timing. Particular weather or climate factors can make the occurrence of an extreme event more predictable. For example, the 2011 drought centered in Texas exhibited relatively high predictability, in large part related to an ongoing La Nina event.
How global warming might affect the likelihood of a drought due to an extreme lack of precipitation is much more difficult to ascertain, because less is known about how global warming affects regional to local rainfall patterns. Over the part of the Central Great Plains that in summer 2012 suffered it's most extreme rainfall deficit since 1895, there is no general scientific agreement on how global warming has affected past rainfall or may affect future rainfall amounts. The recent IPCC SREX report assessed changes in dryness since 1950 and states that there is "medium confidence" of a "decrease in dryness and increase in mean precipitation; large variability dominates" over Central North America (Table 3-2). Current scientific consensus, as expressed in IPCC reports, finds no strong indications for whether rainfall in this region will increase or decrease in the future due to climate change. The Task Force Report considered various additional lines of evidence, including long-term rainfall trends (the last several decades were generally wet) and new climate model simulations with a model that is part of the next IPCC assessment. The latter showed a slight increase in mean summer rainfall due to climate change forcing alone in the Great Plains. Based on multiple lines of evidence, the Report found little support for a systematic effect by global warming on the extreme precipitation deficiencies The Report did find other plausible factors for the precipitation deficiencies linked to natural weather variability. These weather conditions were found to be of a type which historical analysis of past droughts revealed to be quite common for this region.
6. July 2012 was the warmest month on record for the U.S. Did the warm temperatures cause the drought?
Much of the dry region also experienced exceptionally hot temperatures (Fig. 2, in the report). The combination of low rainfall and high temperatures is typically seen during summertime droughts over the central U.S. The historical relationship between rainfall and temperature deficits (Fig. 3 in the report) suggests, however, that 2012 could have been appreciably warmer (perhaps by ~1º C (~1.8º F)) given the severity of rainfall deficits alone. Above-normal temperatures were, however, anticipated in climate models used for prediction, although not the extreme warmth observed which was driven primarily by the absence of rain.
7. How did diminished Arctic sea ice affect the drought?
The model simulations used in the Report included observed sea ice and sea surface temperature conditions. Some researchers have linked Arctic sea ice loss in recent years to unusual summertime jet stream patterns, in effect causing the jet stream pattern to freeze in place and amplify, leading to months-long periods of weather extremes. However, in the summer of 2012, the observed flow did not "freeze in place," but was characterized by substantially different patterns in different months (Fig. 14 in the report). Despite the differences, the patterns all contributed to producing the reduced precipitation that was the primary cause of the drought over the Central Great Plains and adjacent portions of the country during the summer of 2012, but for somewhat different reasons.
8. How did the reduced snow cover in Spring 2012 affect the drought?
The predictions from twelve global forecast centers investigated in the Report were started from initial states based on observed conditions of atmospheric moisture, global circulation, soil moisture, ocean conditions, and snow cover. If those influences strongly constrained summer predictions for US rainfall, it is likely that the predictions would have shown a systematic dry signal over the affected region. The fact that they did not - either when initialized with full Earth system analyses begun in April 2012 or, subsequently, from Earth system analyses in May 2012 when drought conditions were already developing supports a principal conclusion of the Assessment Report that it is very likely that internal atmospheric variability was the principal cause for the severe drought intensity.
9. Does the Report accurately depict the region covered by drought?
The Report is fashioned around national scales of analysis of the 2012 conditions, and is not particularly geared toward a single index of a region. The majority of the graphics (28 plates of figures) given in the Report are for national, hemispheric, and global scales.
For illustrative purposes a six-state region (Wyoming, Colorado, Nebraska, Kansas, Missouri and Iowa) was selected for an in depth look at some aspects of the drought. As can be seen in the figure above, these states comprise a core area of the entire region covered by drought at the end of summer 2012. The Report is quite specific when speaking of a regional event for a specific window of time over which it occurred.
10. Is this Report the final analysis of the 2012 Great Plains Drought?
No, we expect many more investigations into the causes of the 2012 drought. Such investigations are likely to be ongoing for some time; for example, scientists are still investigating the causes of other droughts such as occurred during the 1930's Dust Bowl period. The Report itself lists some limitations of the analysis that require further study. Since the analysis for the Report was completed, additional investigative work has been done by some of the Report's authors have conducted additional research on this event, and a paper has been submitted for peer review and publication in the Bulletin of the American Meteorological Society.
Following the release of the Assessment Report, hypotheses were made by other scientists about other possible factors that may have caused this drought (e.g., deficient Rocky Mountain snow cover, Arctic sea ice loss, greater human-induced global warming influence). More research and peer-reviewed journal articles that explore the influence of such factors on this event would be most welcome to test these hypotheses. Such research will help to push the science forward toward a greater understanding of the nature and causes of droughts and allow for more accurate forecasting and warning of the onset and duration of these phenomena.
--- Content provided by Marty Hoerling, ESRL and Don Murray, ESRL ---