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climate change

Climate change in the news, from my students' perspectives

As scouted out by students in this semester’s Earth Sciences Seminar.

Historical trends, emissions and sinks
Happy 35th birthday, global warming (Real Climate, blogging on peer-reviewed research)

Forest carbon stores may be massively overestimated (New Scientist, UK magazine for science enthusiasts)

Reducing soot might be shortcut to reverse climate change, new study says (Popular Science, US magazine for science enthusiasts)

Weather Extremes
Why was last winter so cold? And is this a problem for climate change? (Outdoor Science, blogging on peer-reviewed research)

2010 Was Fourth Warmest U.S. Summer on
Record
(Science Daily, on-line science press releases)

Weird weather in a warming world (Op-ed section of New York Times, major US newspaper)

Environmental Implications
Arctic species under threat, report warns (CNN.com, major US TV news network)

Too Hot to Handle: Impacts of Climate Change on Mussels (Science Daily, on-line science press releases)

Economic Implications
Climate change could make Canada’s North an economic hothouse (canada.com, an on-line news outlet by Canada’s largest newspaper publisher)

The Promising future of “carbon” farming (National Wildlife Federation, news release)

Report: climate change a factor in world hunger (USA Today, major US newspaper)


The politics of climate change

Republican hopefuls deny global warming (The Guardian, a major UK newspaper)

On the frontline of climate change: “it’s the burning issue Australia’s leaders dare not confront, even on the eve of a general election” (The Independent, a major UK newspaper)

Each student was responsible for bringing in one recent news article,  summarizing it for the class, and for evaluating the effectiveness of the science communication. Students discussed the type of media outlet, intended audience, and how well the writers did in communicating to that audience. They discussed source of information in the article, what was intended to catch the readers’ interest, the use of jargon, and the timeliness of these news pieces.

Heat in the Southeast

Cross-posted at Highly Allochthonous
Here in Charlotte we had a hot summer. We barely escaped the dubious distinction of hottest summer on record, with an average temperature of 81.1° F (27.3 ° C) between 1 June and 31 August. The record had been set in 1993, when Charlotte recorded an average temperature of 81.5° F (27.5 ° C). In terms of record breaking heat, we actually fared better than many parts of the east coast, where temperature records from New York City to Greenville-Spartanburg, South Carolina were broken. Below there’s a nice map from NOAA of how far average temperatures deviated from the 30-year climate normal period (here, 1966-1996).

U.S. surface temperature departure from average (°C), June 1 to August 31, 2010, from NOAA/ESRL Physical Sciences Division, Boulder Colorado

U.S. surface temperature departure from average (°C), June 1 to August 31, 2010, from NOAA/ESRL Physical Sciences Division, Boulder Colorado

Of course those average temperature records belie the minima and maxima experienced by each place over the course of those three summer months, so there’s another statistic that I’m finding even more interesting: the number of days where maximum temperatures exceeded 90° F (32.2 ° C). I think of it as Anne’s index of intolerable heat, especially when combined with the Southeast’s oppressive humidity. In Charlotte, between 1 June and 31 August, we had 67 days that exceeded 90° F. That means that 73% of days this summer were intolerably hot (at least for me). Also, that’s only counting the days in the climatological summer. We had 90+° F degree heat in early April, some in May, and we’ve already had some in September, with more in the forecast this week. I suspect that by the time the year is out, our total days above 90° F will be something around 80, if not more.

The long-term predictions for the index of intolerable heat look grim for Charlotte and the rest of the southeast. The image below shows historical and modeled days with peak temperatures exceeding 90° F. By the end of the century, at least under a high emissions scenario, 80+ days of intolerable heat will be considered a cool summer in North Carolina. We’re heading towards 120 days or more of hot, hot weather, a doubling of our historical average. In parts of Florida and Texas, more than half the year will be hotter than 90° F. Yuck. Glad I won’t be around here then.

Historical and predicted days with peak temperatures above 90 degrees Fahrenheit

These temperature trends are not just bad news for people who like to play (or do field work) outside in the summer, but are too wimpy to drop bucketloads of sweat. Hotter average temperatures and more days with ridiculous heat have real health consequences. On hot days, the chances go up that people playing outside end up with heat exhaustion or life-threatening heat stroke. People without air conditioned homes or workplaces, people too poor to pay tremendous energy bills for air conditioning, or people who just happen to have their AC break do not even need to play outside to be at risk of heat related illness or death. About 700 people already die each year from heat-related causes, and the elderly are a disproportionate share of the victims. Those with cardiovascular disease are also at substantially increased risk of heat-related mortality.

And it’s not the heat alone that spells bad news for the Southeast. With hotter temperatures come increasing rates of photochemical reactions…such as the production of ground-level ozone from nitrous oxides and volatile organic compounds released by car exhaust, power plants, and natural sources. The chemistryof photochemical ozone production is pretty complex and we don’t have a fantastic handle on how coming climate changes will impact the percent of hot days with sun versus clouds, but if the number of hot sunny days increases, it is likely that ozone production will increase too. Ozone brings its own host of adverse health effects, particularly respiratory problems, so even if you don’t mind the heat, running around outside on hot, sunny days can be a bad idea. Once again, children, the elderly, and those with asthma and other respiratory problems are most at risk on high ozone days. Such days, labeled as orange alerts, occur sporadically thoughout the summer already. In Charlotte, we’ve had 13 days with air quality in the orange category since May 1 this year. On those days, people at risk are encouraged to avoid outdoor exercise, and daycare centers limit the time kids spent playing outside. Some days, the air quality is bad enough (red alert) that even healthy adults are encouraged to avoid to outdoor exercise. That’s happened once this year in Charlotte.

As Charlotte and other parts of the southeast move towards one-third of their days in the intolerably hot range, with the probable added bonus of worse air pollution, it will be interesting to watch the societal shifts in attitudes toward the climate. Will Southerners get serious about reducing emissions from cars? Will Charlotteans end their love affair with sprawl in order to improve air quality? Will the Southeast be depopulated of Yankee transplants like me, who finally decide that they can’t take the heat? Or will we just stay inside and crank up the air conditioning units and complain about the weather?

Anne's picks of the June literature: Humans as Agents of Hydrologic Change

ResearchBlogging.orgHow the world’s biggest river basins are going to respond to mid-century climate change…and how large reservoirs affect our measurements of global sea level rise.

Immerzeel, W., van Beek, L., & Bierkens, M. (2010). Climate Change Will Affect the Asian Water Towers Science, 328 (5984), 1382-1385 DOI: 10.1126/science.1183188

Where do 1 in 4 people live? Where do those people get their water? 1.4 billion people live in five river basins (Indus, Ganges, Brahmaputra, Yangtze, and Yellow) and those mighty rivers source some of their water in the Himalayas, where on-going climate change will have a big impact on glacier melt and seasonal precipitation. In this paper, Immerzeel and colleagues used the SRM hydrologic model and GCM outputs to simulate the years 2046-2065 under two different glacier extent scenarios, a “best-guess” and an extreme case where all glacier cover had disappeared. The five basins all behaved quite differently from each other, because each basin has a different topographic distribution. The Brahmaputra and Indus have the highest percent of glacier-covered area, and these two rivers will be the most severely impacted by projected climate change via decreases in late spring and summer streamflow, as reduced glacier melt is only partially offset by increased spring rains. Between these two basins, the authors estimate that the hydrologic changes will reduce the number of people who can be fed by 60 million people! On the other hand, basins with less reliance on meltwater will not be as bad off – in fact, the Yellow River is likely to experience an increase in spring streamflow and may be able to feed 3 million more people. To me this paper emphasizes the fact that the consequences of climate change are not going to be evenly dispensed across the world’s population and that we’ve really got an urgent task of figuring out how regional climate changes will cascade through hydrology, ecology, food security, disease, and almost every other aspect of the world on which we depend.

Fiedler, J., & Conrad, C. (2010). Spatial variability of sea level rise due to water impoundment behind dams Geophysical Research Letters, 37 (12) DOI: 10.1029/2010GL043462

Global reservoirs trap ~10,800 cubic kilometers of water – enough volume to reduce sea level by ~30 mm. But when large reservoirs are filled, the water weight locally depresses the Earth’s surface and increases local relative sea level. Thus, tide gages that are close to large reservoirs don’t record the true sea level effects of water impoundment – instead recording only about 60% of the true drop. This creates an added wrinkle in the estimation of global sea level rise over the last century, and Fiedler and Conrad compute that these reservoir effects on the geoid have caused an ~10% over-estimation in rates of sea level rise. The largest effects on sea level rise records are places where tide gages are near big reservoirs – like the east coast of North America. *

* Please note that I can’t read the full article of AGU publications (including WRR, JGR, and GRL) until July 2010 or the print issue arrives in my institution’s library. Summaries of those articles are based on the abstract only.

Coal, the High Arctic, and the fossil record of climate change

Coals exposed along Stenkul Fiord, southern Ellesmere Island, Canadian Arctic

For more than 55 million years, Ellesmere Island remained in one place while the world around it changed. Fifty-five million years ago, verdant forests grew at 75° North latitude. These wetland forests, [comprised] of species now primarily found in China, grew on an alluvial plain where channels meandered back and forth and periodic floods buried stumps, logs, and leaves intact. Today the forests are preserved as coal seams that outcrop on the edges …[of] modern Ellesmere Island, [where] there are no forests, and the tallest vegetation grows less than 15 cm high. Large parts of the area are polar desert, subject to intensely cold and dark winters and minimal precipitation.

These are the opening lines to my M.S. thesis, in which I contrasted the Paleocene-Eocene and modern hydrological environments of Stenkul Fiord, on southern Ellesmere Island in the Canadian Arctic Archipelago. My thesis goes on to describe a world that no longer exists, except in the fossil record preserved at sites in the High Arctic.  This former world may provide clues as to how polar flora and fauna and their physical environment responded to global mean surface temperatures that were 2-4 degrees warmer than they are today, yet are right in line with the predictions for the end of this century. These clues, recorded in the fossil and stratigraphic record in coal and sediment layers on remote Ellesmere Island, well north of the northernmost civilian settlement in North America, are under attack. The same human demand for energy for that is driving up global temperatures is threatening to erase the very fossils that record polar life under a warmer temperature regime. The government of Canada’s Nunavut territory is currently considering claims by Westar Resources, Inc. to mine the coal beds in one of the most spectacular of all the fossil localities in the High Arctic.

During the Paleocene and Eocene, tropical vegetation extended to 50° N, and broad-leaved evergreens reached 70° N. There was no permanent polar ice, and large parts of the polar regions were covered by forests dominated by cypresses and angiosperms. Fossilized remnants of these forests are found in locations such as Spitsbergen, Greenland, the Yukon, northeastern Asia, and the Canadian Arctic Archipelago. This widespread Arcto-Tertiary forest nearly disappeared as the climate cooled over the past 30 million years and modern temperate forests. Today the last remnants of this flora are preserved in the mountains of China’s Sichuan province.

modern Metasequoia glyptostroboides trunk

Modern Metasequoia glyptostroboides trunk (Image: Wikimedia)

Among the signature trees of the Arcto-Tertiary fossil record is the Metasequoia, a genus which was thought to have gone extinct in the Miocene until an isolated grove of  Metasequoia glyptostroboides, or dawn redwood, was discovered in Sichuan in 1944.  Metasequoia grows to 60 m tall and unlike sequoias, it is deciduous and loses its leaves in the winter.  This would have been quite handy for life in the High Arctic, where in the Paleocene-Eocene winter temperatures might have hovered just above freezing, but would still have been dark for six months of the year.

Metasequoia log, Stenkul Fiord, Ellesmere Island (photo by Anne Jefferson)

Metasequoia log, Stenkul Fiord, Ellesmere Island (photo by Anne Jefferson)

Metasequoia stump, Stenkul Fiord, Ellesmere Island (photo by Anne Jefferson)

Metasequoia stump in its growth position, Stenkul Fiord, Ellesmere Island (photo by Anne Jefferson)

At the site where I worked on Ellesmere Island, there were large Metasequoia logs and tree stumps still rooted in situ in the coal layers. Picking apart the coal layers, I could pull out Metasequoia leaves, twigs, and male and female cones. The siltstones between the coals preserved beautiful fossil impressions of a variety of tree leaves and stems.

An early Eocene tapir fossil from Ellesmere Island (Image courtesy of Jaelyn Eberle)

An early Eocene tapir fossil from Ellesmere Island (Image courtesy of Jaelyn Eberle)

My field site on Stenkul Fiord yielded only plant fossils, and for now, is safe from the development plans of Westar Resources and the Nunavut government. But a bit north at Strathcona Fiord, plants are second fiddle to the best vertebrate fossil locality of the Canadian High Arctic. At Strathcona Fiord,  the fossil record shows that those Eocene forests were inhabited by alligators, giant tortoises, primates, tapirs, and the hippo-like Coryphodon. There have been over 40 papers published on the Eocene fossils of Strathcona Fiord alone. It’s not just the Eocene that makes Strathcona Fiord an amazing fossil locality either. Pliocene layers at Strathcona Fiord have yielded plants, insects, mollusks, fish, frog and mammals such as  black bear, 3-toed horse, beaver,  and badger. It is the only known Pliocene Arctic site with vertebrate remains.

Strathcona Fiord is one of three sites where Westar Resources, Inc. plans to mine the coal. Mining the coal will permanently destroy the embedded fossils and the possibilities for any additional discoveries at this site. The other two Ellesmere Island areas in which Westar has applied for mining permits are the Fosheim and Bache Pennisulas. We don’t know as much about the paleontology of these areas, but the little work that has been done on the Fosheim Peninsula has already discovered Eocene leaf beds and Pliocene fossils.

Paleontologists and geologists around the world are raising their voices in opposition to the proposed coal mining at Strathcona Fiord and the other sites on Ellesmere Island. The Society for Vertebrate Paleontology has issued a press release expressing concern and urging the preservation of the fossils resources. There is also a coordinated letter-writing campaign to the Nunavut Impact Review Board. I’ve just sent a letter to the review board, which I’ve appended below. If you a paleontologist, paleoclimatologist, geologist, Arctic lover, fossil lover, or otherwise moved by the incredible story of alligators and towering trees at 75° N, I urge you join me in writing to the government of Nunavut and encourage them to at least require more study of the localities before mining is approved. Letters can be sent electronically to info@nirb.ca.

To the members of the Nunavut Impact Review Board,

I appreciate the opportunity to write to you concerning the proposed Westar coal project on Ellesmere Island. I am a geologist at the University of North Carolina at Charlotte, and my research focuses on the intersection of hydrology, landscapes, and climate. My graduate M.S. thesis research focused on the paleo-environments of the Eureka Sound Group exposed at Stenkul Fiord on southern Ellesmere Island. I used the coal and sediment layers, and the fossils they contain, to understand variability of hydrological environments that existed in the Arctic 55 million years ago. Today, I work on issues of water and modern climate change, but my perspective was profoundly influenced by the time I spent on Ellesmere Island walking amidst the coal layers and fossilized tree trunks.

The proposed activities by Westar Resources, Inc. could damage or destroy fossil sites that form an important part of Nunavut’s history and environmental legacy. These fossils tell us about the history of Arctic plants and animals, and they are recognized internationally for their scientific importance. They also provide important evidence from a time when Earth, especially the Arctic, was warmer. The fossils of the Ellesmere Island sites proposed for mining by Ellesmere Island provide clues as to how polar flora and fauna and their physical environment responded to global mean surface temperatures that were 2-4 degrees warmer than they are today, yet are right in line with the predictions for the end of this century. Ultimately, I hope that evidence from Nunavut’s fossil record can help us better estimate and prepare for future climate change.

If the fossil sites in the Westar coal project areas are destroyed the evidence is lost forever, therefore I recommend that the Nunavut Impact Review Board advise the Minister, pursuant to article 12.4.4(a) of the Nunavut Land Claim Agreement, that the project proposal requires review under Part 5 or 6. I believe that much more paleontological and paleoclimatic research can be conducted at these sites before any coal is extracted from them and we lose the opportunity to learn all that we can.

I thank you for your consideration, and request that you keep me informed of the results of this screening process.

Post-doctoral Scholar – Oregon State University Hydrogeomorphic response to changing climates in the Pacific Northwest

Described below is a great post-doc opportunity to work with fantastic people. (I should know, I did my PhD and post-doc in this research group.)

We are looking for someone to co-lead a multi-year, inter-institutional research effort to characterize and forecast the effects of changing climate on streamflows and geomorphic processes in the Pacific Northwest. Focus will be on developing and extending theoretical and empirical models of hydrologic response to climate drivers, emphasizing the role of geologic and ecologic controls and filters. The individual hired will have primary responsibility for exploring fruitful lines of attack on the problem, data acquisition and analysis, developing and applying relevant hydrologic and statistical models, and reporting results as journal publications and presentations. This post-doctoral position is with the Watershed Processes Group of Oregon State University (www.fsl.orst.edu/wpg), and the person hired will work closely with federal scientists from the USDA Forest Service Pacific Northwest Research Station.
Qualifications:
1) Ph.D. in hydrology, geomorphology, watershed sciences, or a closely related field, with a demonstrated record of publication or other successful dissemination of work.
2) Strong fundamental understanding of hydrologic processes at the scale of small watersheds to larger catchments, with expertise in one or more of the following: snowpack dynamics, groundwater processes, ecohydrologic interactions, drainage network response to precipitation/runoff relationships.
3) Experience and facility with distributed parameter hydrologic models; familiarity with climate models and climate change scenarios desirable
4) Strong statistics, data analysis and visualization skills, particularly with respect to long time-series data sets.
5) High level working knowledge of GIS and other spatial analysis tools. Expertise with interpreting remote sensing a plus.
Please send a letter of application describing your research experience and qualifications relevant to this position, a complete resume with links to publications, and the names, email addresses and telephone numbers of three references to Sarah Lewis, sarah.lewis@oregonstate.edu or 3200 SW Jefferson Way, Corvallis, Oregon 97330. Review of applications will begin February 15, 2010, and continue until a suitable candidate is found.

My picks of the November literature

It is not that there was no October literature to pick. My time to read articles simply disappeared in the lead-up to and excitement of the Geological Society of America meeting. This month, however, I am back on track and I will try to update this post as I move through the last few weeks of November.

Fussel, H-M. 2009. An updated assessment of the risks from climate change based on research published since the IPCC Fourth Assessment Report. Climatic Change (2009) 97:469–482. doi:10.1007/s10584-009-9648-5
The takeaway message is this: While some topics are still under debate (e.g., changes to tropical cyclones), most recent research indicates that things are looking even worse now than we thought a few years ago. Greenhouse gas emissions are rising faster than we anticipated, and we have already committed to substantial warming, which is currently somewhat masked by high aerosol concentrations. It is increasingly urgent to find mitigation and adaptation strategies. Not good.

Gardner, LR. 2009. Assessing the effect of climate change on mean annual runoff. Journal of Hydrology. 379 (3-4): 351-359. doi:10.1016/j.jhydrol.2009.10.021
This fascinating article starts by showing a strong correlation (r2 = 0.94) between mean annual runoff and a function of potential evapotranspiration and precipitation. The author then goes on to derive an equation that shows how temperature increases can be used to calculate the change in evapotranspiration, therefore solving the water budget and allowing the calculation of the change in mean annual runoff. Conversely, the same equation can be used to solve for the necessary increase in precipitation to sustain current runoff under different warming scenarios.

Schuler, T. V., and U. H. Fischer. 2009.Modeling the diurnal variation of tracer transit velocity through a subglacial channel, J. Geophys. Res., 114, F04017, doi:10.1029/2008JF001238.
The authors made multiple dye tracer injections into a glacial moulin and then measured discharge and tracer breakthrough at the proglacial channel. They found strong hysteresis in the relationship between tracer velocity and proglacial discharge and attributed this hysteresis to the adjustment of the size of a subglacial Röthlisberger channel to hydraulic conditions that change over the course of the day. Cool!

Bense, V. F., G. Ferguson, and H. Kooi (2009), Evolution of shallow groundwater flow systems in areas of degrading permafrost, Geophys. Res. Lett., 36, L22401, doi:10.1029/2009GL039225.
Warming temperatures in the Arctic and sub-arctic are lowering the permafrost table and activating shallow groundwater systems, causing increasing baseflow discharge of Arctic rivers. This paper shows how the groundwater flow conditions adjust to lowering permafrost over decades to centuries and suggests that even if air temperatures are stabilized, baseflow discharge will continue to increase for a long time.

Soulsby, Tetzlaff, and Hrachowitz. Tracers and transit times: Windows for viewing catchment scale storage. Hydrological Processes. 23(24): 3503 – 3507. doi: 10.1002/hyp.7501
In this installment of Hydrological Processes series of excellent invited commentaries, Soulsby and colleagues remind readers that although flux measurements have been the major focus of hydrologic science for decades, it is storage that is most relevant for applied water resources problems. They show that tracer-derived estimates of mean transit time combined with streamflow measurements can be used to calculate the amount of water stored in the watershed. They use their long-term study watersheds in the Scottish Highlands to illustrate how transit time and storage scale together and correlate with climate, physiography, and soils in the watersheds. Finally, they argue that while such tracer-derived storage estimates have uncertainties and are not a panacea, they do show promise across a range of scales and geographies.

Chatanantavet, P., and G. Parker (2009), Physically based modeling of bedrock incision by abrasion, plucking, and macroabrasion, J. Geophys. Res., 114, F04018, doi:10.1029/2008JF001044.
Over the past 2 decades, geomorphologists have developed much better insight into the landscape evolution of mountainous areas by developing computerized landscape evolution models. A key component of such models is the stream power rule for bedrock incision, but some have complained that is not physically based enough to describe. In this paper, the authors lay out a new model for bedrock incision based on the mechanisms of abrasion, plucking, and macroabrasion (fracturing and removal of rock by the impact of moving sediment) and incorporating the hydrology and hydraulics of mountain rivers. This could be an influential paper.

Payn, R. A., M. N. Gooseff, B. L. McGlynn, K. E. Bencala, and S. M. Wondzell (2009), Channel water balance and exchange with subsurface flow along a mountain headwater stream in Montana, United States, Water Resour. Res., 45, W11427, doi:10.1029/2008WR007644.

Tracer tests were conducted along 13 continuous reaches of a mountain stream to quantify gross change in discharge versus net loss and net gain. Interestingly, the change in discharge over some reaches did not correspond to calculations of net loss or net gain based on tracer recovery. These results suggests that commonly used methods for estimating exchange with subsurface flow may not be representing all fluxes. Bidirectional exchange with the subsurface, like that found in this paper, is likely to be very important for nutrient processing and benthic ecology.

Please note that I can’t read the full article of AGU publications (including WRR, JGR, and GRL) until July 2010 or the print issue arrives in my institution’s library. Summaries of those articles are based on the abstract only.

Glacier loss and the consequences for streamflow and sea levels

On an afternoon when I was working through some edits on a manuscript detailing the role of glacier melt water in sustaining agricultural water supplies in the Hood River valley of Oregon, it was more than appropriate that my good friend Chris Rowan should send me the link this beautiful and scary TED talk on photographic evidence of rapid ice loss from glaciers around the world..

How much are those retreating glaciers going to add to global sea levels under projected warming scenarios? By one recent estimate, up to 2 m by the end of the century, though more realistically 0.8 m in that time frame. Not such good news for those like to vacation on the Outer Banks or enjoy the majestic beauty of glaciers capping Oregon’s volcanoes.

A few semantics about climate variability and change

Last week, the Southeastern United States received several inches of snow. This late season snowfall was certainly a novelty, though not an unprecedented occurrence. But it did stir up conversations among local residents, especially when the week ended with ~25 degree Celsius (75 Fahrenheit) sunshine. The weather’s fickleness also got me thinking about climate variability and climate change and how easily we can slip up and confuse the two. I even see scientists (who should know better) conflating variability and change, so below I offer a short, illustrated tutorial on the differences.

Hydrometeorological variables are things like precipitation, streamflow, groundwater levels, temperature, and humidity and are often expressed as annual or seasonal averages. The average value of one of those variables over 30 years is called a climatological normal. Below, I’ve illustrated a hypothetical climate variable as it varies of a 30 year period. These normals are redefined every 10 years, so right now we are using 1971-2000 as our normal period.

An example (hypothetical) climate variable through time

Figure 1. A hypothetical climate variable through time

The average value of the variable is 0.5, and the squiggles above and below the mean represent climate variability. I’ll define climate variability as the oscillations around a mean state. (An aside: it’s fairly common to see a few years in a row that are below the mean or above the mean, in a phenomenon known as serial correlation, where the value of a variable is influenced by the values that precede it. As an example, if you have a severe drought one year, even if it rains more than normal the next year, streamflow may stay quite low as groundwater is replenished. This is what is happening in the southeast now after our 2007 drought.)

Variability then is all about the oscillations, but it doesn’t tell you anything about what’s happening with the mean. Below, I’ve illustrated the same time series shifted progressively by 0.003 per time step. Here the mean is changing, while the variability stays the same.

A (hypothetical) climate variable in blue is trending by 0.003 per year (with the non-trending time series in gray for comparison)

Figure 2. A hypothetical climate variable in blue is trending by 0.003 per year (with the non-trending time series in gray for comparison)

As in the illustration above, variables like average temperature and sea surface temperature are experiencing changes in their mean values. So, climate change can take the form of a trend in the mean value of a variable over time. A climatological variable experiencing change in the mean would not have the same “normal” values from one climate normal period to the next.

But climate change can also affect the variability of a variable, as illustrated below. Here the mean is not changing, but I’ve made below-mean points successively lower by 0.0067 per time step and above mean points are successively higher by 0.00347 per time step.

A hypothetical climate variable (blue) showing an increase in variability with time (gray line is the variable with unchanging mean and variability)

Figure 3. A hypothetical climate variable (blue) showing an increase in variability with time (gray line is the variable with unchanging mean and variability)

This sort of change is the sort of change we might see in precipitation in some areas. For example, the Southeastern United States is predicted to have more intense summer rainfall and more intense droughts, and retrospective trend studies suggest that this may already be the case. Even though the mean precipitation is not changing, the Southeastern United States is still experiencing a climate change effect manifested in a change in climate variability.

Finally, climate change can take the form of a trend in the mean and a trend in the variability, as shown below.

A hypothetical climate variable with changing mean and variability (gray solid line indicates variable with unchanging mean and variability, gray dotted line has a changing mean without changing variability)

Figure 4. A hypothetical climate variable with changing mean and variability (gray solid line indicates variable with unchanging mean and variability, gray dotted line has a changing mean without changing variability)

This final pattern may be the case for streamflow in some regions. Mean streamflow could decrease because of increasing evapotranspirative losses in a warmer climate, and streamflow variability could increase because of changes in precipitation and drought intensity. This sort of complicated pattern may occur for other climatological variables as well.

So what does this mean for “freak” late winter snowstorms in the southeastern United States? Climate change trending towards warmer temperatures makes frozen precipitation less likely (Figure 2), but given the variability inherent in meteorological systems (Figure 1), I wouldn’t rule it out entirely. But the snowshoes in my garage are still feeling a bit neglected.

AGU Abstract Submitted: Secular Streamflow Trends in Watersheds Receiving Mixed Rain and Snow, Pacific Coast and Cascades Ranges

The following abstract was submitted for the Fall AGU meeting:

Secular Streamflow Trends in Watersheds Receiving Mixed Rain and Snow, Pacific Coast and Cascades Ranges

A. Jefferson, University of North Carolina at Charlotte

Much existing research has focused on detecting climate change effects on snowmelt-dominated watersheds, but in the Pacific Coast and Cascades ranges, precipitation falls as either rain or snow, depending on latitude, elevation, and season. Watersheds often straddle the snow line, with some areas dominantly receiving rain and higher elevations accumulating seasonal snowpacks. These snowpacks are near the 0°C threshold, making them sensitive to the effects of climate warming. Climate sensitivity of seasonal and event hydrographs from watersheds with mixed rain and snow has not been fully explored. This project investigates detectable climate change signals in long-term streamflow records in the Washington, Oregon, and northern California Coast and Cascades Ranges.

Watersheds with mean elevations above the seasonal snow line show significant increases in streamflow during January through March and decreases in the percent of annual flow during April through June, the historical snowmelt period. These changes were not detectable in watersheds with mean elevations below the seasonal snow line. There were no consistent trends in peakflow dates or volumes. The multiple drivers of peakflow occurrence make it unlikely that any changes in peakflow timing will be detectable for several decades. Results suggest that Coast Range hydrology has been minimally impacted by historical climate warming, but that Cascades Range watersheds are already experiencing altered hydrologic regimes.

Pending acceptance, the work will be presented in session H32 Spatial and Temporal Trends in Hydrometeorological Records as Indicators of Climate Variability and Change.