by Judith Curry
Imagine that you are planning for water resources in the greater Atlanta region for the next 50 years. Which information would be more helpful:
A. Global climate model simulations for the 21st century that are downscaled for the region
B. Paleoclimatic analysis for the last 400 years of droughts and wet periods.
The CMIP3 climate models used in the AR4 basically tell us nothing about future rainfall in the 21st century for this region, other than that the different model runs don’t agree.
If you are wondering what we might learn from B, take a look at this recent article.
A long-term perspective on a modern drought in the American southeast
N Pedersen, AR Bell, TA Knight, C Leland, N Malcomb, KJ Anchukaitis, K Tackett, J. Cheff, A Brice, B Catron, W Blozan, J. Riddle
Abstract. The depth of the 2006–9 drought in the humid, southeastern US left several metropolitan areas with only a 60–120 day water supply. To put the region’s recent drought variability in a long-term perspective, a dense and diverse tree-ring network—including the first records throughout the Apalachicola–Chattahoochee–Flint river basin—is used to reconstruct drought from 1665 to 2010 CE. The network accounts for up to 58.1% of the annual variance in warm-season drought during the 20th century and captures wet eras during the middle to late 20th century. The reconstruction shows that the recent droughts are not unprecedented over the last 346 years. Indeed, droughts of extended duration occurred more frequently between 1696 and 1820. Our results indicate that the era in which local and state water supply decisions were developed and the period of instrumental data upon which it is based are amongst the wettest since at least 1665. Given continued growth and subsequent industrial, agricultural and metropolitan demand throughout the southeast, insights from paleohydroclimate records suggest that the threat of water-related conflict in the region has potential to grow more intense in the decades to come.
Citation: Environ. Res. Lett. 7 (2012) 014034 (8pp) [link] to abstract (if you would like a copy of the paper, send me an email:
The Introduction provides some context for the decision making problem:
Drought is a pervasive phenomenon throughout much of North America with profound ecological and societal implications. Although much attention has been devoted to forecasting the frequency and magnitude of drought in semi-arid western North America, recent moisture deficits in the southeastern US have renewed water management challenges that underscore the need to better understand drought processes in humid, subtropical regions. Notably, the droughts of 1986–8, 1998–2002 and 2006–9 caused severe crop damage, disruptions in electricity generation and water shortages, which prompted water restrictions and multi-state legal conflicts . This is particularly evident in the state of Georgia, where droughts during the 1980s and 1990s occurred concomitant with a 50% increase in population and a 35% increase in groundwater withdrawal. During the 2006–9 drought, many municipal water supplies throughout the region, including Atlanta, were reduced to 60–120 day capacities.
Given recent water shortages and emerging challenges, Georgia and adjacent states have revised water management plans to include greater focus on conservation and efficiency. Unfortunately, many water allocation plans are based on limited 20th century records and capture a narrow range of potential moisture variability. To plan for an expanded range of natural and anthropogenically forced variability, water managers have begun to incorporate tree-ring based hydroclimate reconstructions to place recent droughts in a long-term context. Tree-ring based perspectives suggest that the 20th century has been relatively moist with respect to the last millennia in the eastern US and that although recent droughts have had significant societal implications, they are in most cases less severe relative to prior centuries.
From the Conclusion:
The latter 20th century instrumental data, upon which regional water supply management decisions are based, is characterized by frequent wet events that are not representative of much of the prior 300 yr. Investigations of long-term drought in other regions of the southeastern US have similar findings: the 20th century appears wetter in the context of the last 400–1000 years…..The climatic patterns revealed here—the pervasively drier 18th century, the weak wet periods of the 19th century, and the high frequency of extreme drought in the early 18th and 19th centuries—provide valuable baseline scenarios for simulation of inter-annual climate variability and water resources planning that do not appear in the more recent, relatively wetter instrumental records. Although non-stationarity of the climate system could cause climate variability to differ from what has occurred historically …..this reconstruction provides a broader representation of the potential range of climate variability than is available from the instrumental record alone, and thus is a valuable tool for understanding the context of extreme events to which our infrastructure must be able to adapt.
JC comment: IMO, studies such as this provide much more useful information for regional decision making than do climate models. The amount of rainfall variability on decadal and multi-decadal time scales is far greater than anything that has emerged from 21st century climate model simulations.
Interesting that this paper uses tree rings to infer rainfall rather than temperature. As far as I can tell, the authors have a very good reputation and the methods used in this paper are sound.