B24B-01 INVITED
Hydrometeorological Extremes: Occurrence And Forcing Factors
One of the most critical aspects of our present and future climate is the occurrence of extremes. These events often lead to some of the greatest impacts from climate and this certainly applies to hyrometeorological extremes. Such extremes include drought, heavy precipitation and flooding as well as combinations of such phenomena since drought is sometimes interwoven with major precipitation events. Hydrometeorological extremes also include associated phenomena such as rain-on-snow events, hazardous freezing precipitation, and the atypical timing of such events. From the point of view of the cycling of water, such phenomena, although quite distinct, share many features. They are formed thorough a combination of large scale processes (circulations, storm tracks), atmospheric processes over particular regions (water vapour, clouds, precipitation), and surface processes over such regions (evaporation, sublimation, soil moisture, vegetation, and terrain) and they are often closely linked to temperature values and thresholds. Key scientific issues are concerned with how such processes sometimes produce extremes, with the possible paths through which extremes can be produced and sustained, and with the means by which they can end. This presentation focuses on the factors associated with such extremes including their future occurrence. It in particular incorporates some of the research activities being undertaken globally and regionally within the Global Energy and Water Cycle Experiment (GEWEX). Such studies are providing us with a greater awareness of the common and distinct features of hydrometeological extremes in different regions of the world. All this work is contributing to an improving appreciation of the future occurrence of such extremes and therefore it will contribute to better understanding impacts including those affecting the biosphere.
B24B-02 INVITED
Recent Extremes in European Climate: Assessment, Case Studies and Impacts
During the last centuries and up to the present decade, extreme climate events have certainly had larger economic impacts than any trend of temperature in Europe. In addition to an intrinsic scientific interest, their study is thus essential for society. One of the challenges of their investigation is that, depending on their definition, extreme climate events potentially have a behavior that is not connected to the secular temperature trend in a simple fashion. This presentation will review the statistical assessments of extremes in Europe, focusing on surface temperature, precipitation, and their connections with large-scale features of the atmospheric circulation. In particular, the questions of modeling their severity and frequency will be discussed in the first part of the presentation. I will then give two kinds of examples of European climate extremes: summer heatwaves and droughts, and winter warm waves. The mechanisms leading to such phenomena will be explored, and I will examine some of the impacts on the biosphere that were recently observed. In order to provide a long term perspective of those events, examples of historical droughts in France will be presented and connected with proxy records of temperature. It appears that the mechanisms that are favored for present-day climate might still have been valid during the past centuries. To conclude, new challenges for dynamical and statistical modeling will be explored.
B24B-03 INVITED
Overview of the biology of extreme events
Extreme events have, variously, meteorological origins as in heat waves or precipitation extremes, or biological origins as in pest and disease eruptions (or tectonic, earth-orbital, or impact-body origins). Despite growing recognition that these events are changing in frequency and intensity, a universal model of ecological responses to these events is slow to emerge. Extreme events, negative and positive, contrast with normal events in terms of their effects on the physiology, ecology, and evolution of organisms, hence also on water, carbon, and nutrient cycles. They structure biogeographic ranges and biomes, almost surely more than mean values often used to define biogeography. They are challenging to study for obvious reasons of field-readiness but also because they are defined by sequences of driving variables such as temperature, not point events. As sequences, their statistics (return times, for example) are challenging to develop, as also from the involvement of multiple environmental variables. These statistics are not captured well by climate models. They are expected to change with climate and land-use change but our predictive capacity is currently limited. A number of tools for description and analysis of extreme events are available, if not widely applied to date. Extremes for organisms are defined by their fitness effects on those organisms, and are specific to genotypes, making them major agents of natural selection. There is evidence that effects of extreme events may be concentrated in an extended recovery phase. We review selected events covering ranges of time and magnitude, from Snowball Earth to leaf functional loss in weather events. A number of events, such as the 2003 European heat wave, evidence effects on water and carbon cycles over large regions. Rising CO2 is the recent extreme of note, for its climatic effects and consequences for growing seasons, transpiration, etc., but also directly in its action as a substrate of photosynthesis. Effects on water and N cycles are already marked. Adaptive responses of plants are very irregularly distributed among species and genotypes, most adaptive responses having been lost over 20 My of minimal or virtually accidental genetic selection for correlated traits. Offsets of plant activity from those of pollinators and pests may amplify direct physiological effects on plants. Another extreme of interest is the insect-mediated mass dieoff of conifers across western North America tied to a rare combination of drought and year-long high temperatures.
B24B-04
Drought Effects on Ecosystem-Atmosphere Exchanges Detected with FLUXNET
Droughts and heat waves stress ecosystems with high temperatures, dry soils, or high evaporative demand. Severe droughts tend to diminish gross and net primary productivity by inducing stomatal limitation, while ecosystem respiration can either increase or decrease. The FLUXNET network presents an excellent opportunity for examining coherence in functional responses of carbon and water fluxes to drought. We conduct across-site, integrated analysis of daily, monthly, and composite diurnal carbon and water fluxes during warm season droughts. Daily net ecosystem CO2 exchanges were generally reduced by summer droughts. This is associated with slightly decreased respiration but greater reduction in productivity. In contrast, some sites show offsetting reductions of respiration and productivity leading to little difference in total daily net ecosystem exchange. Evapotranspiration is also generally reduced by drought, and reductions of daytime CO2 flux and evapotranspiration are similarly scaled. However, flux responses are not consistently proportional to the magnitude of drought recorded in the Palmer Drought Index. This underscores how drought effects on ecosystem carbon and water fluxes are a mixture of process-level responses active at various timescales, warranting continued study integrating across a wide sample of sites and events.
B24B-05
Carry-over effect of a series of anomalous dry years on carbon and water exchange in a semi-arid Ponderosa Pine forest in the Pacific Northwest
The effect of a series of anomalous dry years (2001 – 2003) was investigated in the frame of continuous carbon, water and energy exchange measurements using the eddy covariance technique in a mature ponderosa pine forest over a period of 6 years (2002-2007). The annual precipitation in the series of dry years was -227 (-38%), -71 (-12%) and -197 mm (-33%) lower than the 26 year climatological average (598 mm) from 1982-2007 in hydrological years, and therefore represents an extreme climate anomaly. Summer droughts, which are common in this region, are generally attributed to larger- regional circulation patterns, and usually prevail from mid-summer until late fall. Even though these forests have adapted to drought, the extreme series of dry years resulted in the lowest annual NEE in 2003 of -263 gC m-2 compared to the six year average of -443±110 gC m-2. The response was likely caused by carry-over effects of reduced carbohydrate reserves in plant tissue and lack of soil moisture recharge at greater soil depths. Also, annual litterfall reached a maximum in 2003, suggesting that trees in years following the dry period were not able to support/maintain a canopy similar to that of previous years. Total annual evapotranspiration in 2003 was also reduced by 14% compared to the 6 year average and thus confirmed the severe effects of the extreme multi-year drought. Individual dry years (e.g. 2001) were not found to have the same impact on annual carbon and water budgets, suggesting that the response of a summer drought-adapted ecosystem to multi-year climate extremes is highly non-linear. This observation calls for the necessity of accounting for carry-over effects of multi-year climate anomalies in ecosystem- and regional-scale carbon and carbon-water coupled models. Often these models run for calendar year periods starting with fully recharged soil moisture profiles and carbon pools and explicitly do not allow for cumulative effects on physiological processes.
B24B-06
Impact of seasonal and annual climate variability and extreme weather events on carbon exchanges in an age-sequence of temperate pine forest in Canada
We present five years (2003-2007) of eddy covariance flux measurements made in an age-sequence (68-,
33-, 18-, and 5-year-old) of eastern white pine (-Pinus strobus L.) forests in southern Ontario, near Lake
Erie in Canada. The goal of this study was to evaluate the impact of seasonal and annual variations in
climatic variables and extreme weather events on gross primary productivity (GEP), ecosystem respiration
(RE) and net ecosystem productivity (NEP) of all four stands. Five-year mean NEP values were 136 (36 to
219), 442 (318 to 666), 774 (684 to 885) and 40 (-126 to 164) g C m -2 y -1 in the 68-, 33-, 18-,
and 5-year-old stand, respectively. The study period experienced three distinct extreme weather events:
warm and dry spring of 2005, extremely wet summer of 2006 and summer drought of 2007. In 2005, NEP of
68-year-old stand was reduced to 36 g C m -2 y -1 (74% reduction) as compared to its five-year
mean value, mostly because of decrease in photosynthetic activity due to an early spring drought. In 2005,
GEP and RE values were 1237 and 1176 g C m -2 y -1 as compared to respective five-year mean
GEP and RE values of 1349 and 1189 g C m -2 y -1. Similar decrease in NEP was observed at the
33-year-old site, while the 5-year-old site became a large source of carbon. The 18-year-old stand which had
high water table due to its topography was least affected. In contrast, 2006 was the most productive year,
with highest ever recorded GEP value of 1468 g C m -2 y -1 in the 68-year-old stand but hot
summer temperatures resulted in highest RE as well (1292 g C m -2 which was 103 g C m -2 more
compared to five-year mean value), effectively making 2006 an average year in terms of annual NEP. In
2007, a summer drought caused GEP to decline but RE was similar to its five-year mean value due to colder
temperatures in late summer and autumn, resulting in NEP similar to its five-year mean value. The most
productive year was 2003, when low temperature and RE resulted in 219 g C m -2 y -1 of NEP. The
results of this study indicate that carbon cycle of forests in great lakes region in eastern North America might
be more affected by changes in temperature rather than variations in precipitation, unless dry conditions
coincide with short and intense growing season of the region. These results also highlight the complexity of
forest ecosystem responses to climatic and extreme weather events.
http://www.science.mcmaster.ca/geo/faculty/arain/research/index.htm
B24B-07
Sensitivity of Soil Carbon Balances to Changes in the Extent and Duration of Soil Frost in a Boreal Forest
Climate change is likely to alter soil frost depth and duration in the boreal zone in the future. Soil frost can influence biogeochemical soil processes directly, for instance by altering rates of microbial decomposition of soil organic matter (SOM) and by increasing root mortality. In addition, soil frost controls hydrologic flow paths, which in turn determines export of carbon from soils to surface waters, primarily during high-flood events associated with snow melt and soil thawing. Nonetheless, to what extent changes in soil frost regimes influences the soil C dynamics is poorly understood. Here we present results from a field manipulation investigation with three soil frost treatments (deep soil frost, shallow soil frost and ambient controls; n = 3) that has been in operation for 7 years. Increased soil frost depth results in decreased soil CO2 concentrations and soil respiration rates during the following growing season. We see a strong correlation between the maximum soil frost depth during winter and the amount of C lost from the system by soil respiration (R = 0.99) suggesting that average soil respiration rates during the growing season will decrease by ca 0.01 g CO2 m-2 day-1 for every 1 cm increase in soil frost depth. This corresponds to up to 0.5% of the estimated annual net ecosystem productivity. Because year-to-year variation in soil frost depth at the site varies by up to 60 cm, we conclude that it can constitute an important control on soil C balances. Results also show a significant effect on the pool of dissolved organic carbon (DOC) in the soil during spring thaw and early summer, where deep soil frost treatments show up to twice the amount of DOC, as compared to shallow soil frost treatments. In addition, laboratory incubations suggest that alterations in winter soil temperatures and soil frost distribution also affects the composition of the DOC pool altering its aromaticity, with potential effects on its bioavailability. Thus, a change in soil frost regime has implications for both the amount and the composition of C exported from soils to surface waters.
B24B-08
Changes in Net Ecosystem Productivity of Boreal Black Spruce Stands in Response to Changes in Temperature at Diurnal and Seasonal Time Scales
Net ecosystem productivity (NEP) of boreal coniferous forests is believed to rise with climate warming, thereby offsetting some of the rise in atmospheric CO2 concentration (Ca) by which warming is caused. However the response of conifer NEP to warming may vary seasonally, with rises in spring and declines in summer. To gain more insight into this response, we compared changes in CO2 exchange measured by eddy covariance (EC) and simulated by the ecosystem process model ecosys under rising mean annual air temperatures (Ta) during 2004--2006 at black spruce stands in Saskatchewan, Manitoba and Quebec. Hourly net CO2 uptake was found to rise with warming at Ta < 15° C, and to decline with warming at Ta > 20° C. As mean annual Ta rose from 2004 to 2006, increases in net CO2 uptake with warming at lower Ta were greater than declines with warming at higher Ta so that annual gross primary productivity (GPP) and hence NEP increased. Increases in net CO2 uptake measured at lower Ta were explained in the model by earlier recovery of photosynthetic capacity in spring, and by increases in carboxylation activity, using parameters for Arrhenius temperature functions of key carboxylation processes derived from independent experiments. Declines in net CO2 uptake measured at higher Ta were explained in the model by sharp declines in midafternoon canopy conductance (gc) under higher vapour pressure deficits (D). These declines were modelled from a hydraulic constraint to water uptake imposed by low axial conductivity of conifer roots and boles that forced declines in canopy water potential (ƒéc) and hence in gc under higher D when equilibrating water uptake with transpiration. In a model sensitivity study, the contrasting responses of net CO2 uptake to specified rises in Ta caused annual NEP of black spruce in the model to rise with increases in Ta of up to 6° C, but to decline with further increases at mid-continental sites with lower precipitation. However these contrasting responses to warming also indicate that rises in NEP with climate warming would depend on the seasonality (spring vs. summer) as well as the magnitude of rises in Ta.