Weather Abnormalities in Present and Future Climate

Via Prof Paul Vincelli of the University of Kentucky, a list of recent papers in addition to HSR on the anomalous warm events of recent years.

Christidis, N., P.A. Stott, and S. Brown, 2011: The role of human activity in the recent
warming of extremely warm daytime temperatures. Journal of Climate Vol. 24,
doi:10.1175/2011JCLI4150.1.

Formal detection and attribution analyses of changes in daily extremes give evidence of a significant human influence on the increasing severity of extremely warm nights and decreasing severity of extremely cold days and nights. This paper presents an optimal fingerprinting analysis that also detects the contributions of external forcings to recent changes in extremely warm days using nonstationary extreme value theory. The authors’ analysis is the first that attempts to partition the observed change in warm daytime extremes between its anthropogenic and natural components and hence attribute part of the change to possible causes. Changes in the extreme temperatures are represented by the temporal changes in a parameter of an extreme value distribution. Regional distributions of the trend in the parameter are computed with and without human influence using constraints from the global optimal fingerprinting analysis. Anthropogenic forcings alter the regional distributions, indicating that extremely warm days have become hotter.

2. Coumou and Rahmstorf 2012. A decade of weather extremes. Nature Climate Change,
Vol. 2. DOI: 10.1038/NCLIMATE1452.

The ostensibly large number of recent extreme weather events has triggered intensive discussions, both in- and outside the scientific community, on whether they are related to global warming. Here, we review the evidence and argue that for some types of extreme — notably heatwaves, but also precipitation extremes — there is now strong evidence linking specific events or an increase in their numbers to the human influence on climate. For other types of extreme, such as storms, the available evidence is less conclusive, but based on observed trends and basic physical concepts it is nevertheless plausible to expect an increase.

3. Dai, A. 2010. Drought under global warming, a review. WIRES Climate Change 2:45-65.

This article reviews recent literature on drought of the last millennium, followed by an update on global aridity changes from 1950 to 2008. Projected future aridity is presented based on recent studies and our analysis of model simulations. Dry periods lasting for years to decades have occurred many times during the last millennium over, for example, North America, West Africa, and East Asia. These droughts were likely triggered by anomalous tropical sea surface temperatures (SSTs), with La Niña-like SST anomalies leading to drought in North America, and El-Niño-like SSTs causing drought in East China. Over Africa, the southward shift of the warmest SSTs in the Atlantic and warming in the Indian Ocean are responsible for the recent Sahel droughts. Local feedbacks may enhance and prolong drought. Global aridity has increased substantially since the 1970s due to recent drying over Africa, southern Europe, East and South Asia, and eastern Australia. Although El Niño-Southern Oscillation (ENSO), tropical Atlantic SSTs, and Asian monsoons have played a large role in the recent drying, recent warming has increased atmospheric moisture demand and likely altered atmospheric circulation patterns, both contributing to the drying. Climate models project increased aridity in the 21st century over most of Africa, southern Europe and the Middle East, most of the Americas, Australia, and Southeast Asia. Regions like the United States have avoided prolonged droughts during the last 50 years due to natural climate variations, but might see persistent droughts in the next 20–50 years. Future efforts to predict drought will depend on models’ ability to predict tropical SSTs.

4. Dai, A. 2012. Increasing drought under global warming in observations and models.
Nature Climate Change DOI:10.1038/NCLIMATE1633

Historical records of precipitation, streamflow and drought indices all show increased aridity since 1950 over many land areas1, 2. Analyses of model-simulated soil moisture3, 4, drought indices1, 5, 6 and precipitation-minus-evaporation7 suggest increased risk of drought in the twenty-first century. There are, however, large differences in the observed and model-simulated drying patterns1, 2, 6. Reconciling these differences is necessary before the model predictions can be trusted. Previous studies8, 9, 10, 11, 12 show that changes in sea surface temperatures have large influences on land precipitation and the inability of the coupled models to reproduce many observed regional precipitation changes is linked to the lack of the observed, largely natural change patterns in sea surface temperatures in coupled model simulations13. Here I show that the models reproduce not only the influence of El Niño-Southern Oscillation on drought over land, but also the observed global mean aridity trend from 1923 to 2010. Regional differences in observed and model-simulated aridity changes result mainly from natural variations in tropical sea surface temperatures that are often not captured by the coupled models. The unforced natural variations vary among model runs owing to different initial conditions and thus are irreproducible. I conclude that the observed global aridity changes up to 2010 are consistent with model predictions, which suggest severe and widespread droughts in the next 30–90 years over many land areas resulting from either decreased precipitation and/or increased evaporation.

5. Duffy, P. B., and C. Tebaldi. 2012. Increasing prevalence of extreme summer
temperatures in the U.S. Climatic Change, Vol. 111. DOI 10.1007/s10584-012-0396-6.

Human-caused climate change can affect weather and climate extremes, as well as mean climate properties. Analysis of observations and climate model results shows that previously rare (5th percentile) summertime average temperatures are presently occurring with greatly increased frequency in some regions of the 48 contiguous United States. Broad agreement between observations and a mean of results based upon 16 global climate models suggests that this result is more consistent with the consequences of increasing greenhouse gas concentrations than with the effects of natural climate variability. This conclusion is further supported by a statistical analysis based on resampling of observations and model output. The same climate models project that the prevalence of previously extreme summer temperatures will continue to increase, occurring in well over 50% of summers by mid-century.

6. Francis, J. and S. Vavrus, 2012. Evidence linking Arctic amplification to extreme weather
in mid-latitudes. Geophysical Research Letters Vol. 39, L06801, doi:10.1029/
2012GL051000.

Arctic amplification (AA) – the observed enhanced warming in high northern latitudes relative to the northern hemisphere – is evident in lower-tropospheric temperatures and in 1000-to-500 hPa thicknesses. Daily fields of 500 hPa heights from the National Centers for Environmental Prediction Reanalysis are analyzed over N. America and the N. Atlantic to assess changes in north-south (Rossby) wave characteristics associated with AA and the relaxation of poleward thickness gradients. Two effects are identified that each contribute to a slower eastward progression of Rossby waves in the upper-level flow: 1) weakened zonal winds, and 2) increased wave amplitude. These effects are particularly evident in autumn and winter consistent with sea-ice loss, but are also apparent in summer, possibly related to earlier snow melt on high-latitude land. Slower progression of upper-level waves would cause associated weather patterns in mid-latitudes to be more persistent, which may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves.

7. Hansen, J., M. Satoa, and R. Ruedy, 2012. Perception of climate change. Proceedings of
the National Academy of Science. doi/10.1073/pnas.1205276109.

“Climate dice,” describing the chance of unusually warm or cool seasons, have become more and more “loaded” in the past 30 y,
coincident with rapid global warming. The distribution of seasonal mean temperature anomalies has shifted toward higher temperatures
and the range of anomalies has increased. An important change is the emergence of a category of summertime extremely
hot outliers, more than three standard deviations (3σ) warmer than the climatology of the 1951–1980 base period. This hot extreme,
which covered much less than 1% of Earth’s surface during the base period, now typically covers about 10% of the land area. It follows
that we can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and
Moscow in 2010 were a consequence of global warming because their likelihood in the absence of global warming was exceedingly
small. We discuss practical implications of this substantial, growing, climate change.

8. Meehl, G. A., C. Tebaldi, G. Walton, D. Easterling, and L. McDaniel, 2009. Relative
increase of record high maximum temperatures compared to record low minimum
temperatures in the U.S., Geophysical Research Letters Vol. 36, L23701, doi:10.1029/
2009GL040736.

The current observed value of the ratio of daily record high maximum temperatures to record low minimum temperatures averaged across the U.S. is about two to one. This is because records that were declining uniformly earlier in the 20th century following a decay proportional to 1/n (n being the number of years since the beginning of record keeping) have been declining less slowly for record highs than record lows since the late 1970s. Model simulations of U.S. 20th century climate show a greater ratio of about four to one due to more uniform warming across the U.S. than in observations. Following an A1B emission scenario for the 21st century, the U.S. ratio of record high maximum to record low minimum temperatures is projected to continue to increase, with ratios of about 20 to 1 by mid-century, and roughly 50 to 1 by the end of the century.

9. Peterson, T., X. Zhang, M. Brunet-India, and J. L. Vázquez-Aguirre 2008. Changes in
North American extremes derived from daily weather data. Journal of Geophysical
Research Vol. 113. doi:10.1029/2007JD009453.

Detailed homogeneity assessments of daily weather observing station data from Canada, the United States, and Mexico enabled analysis of changes in North American extremes starting in 1950. The approach used a number of indices derived from the daily data, primarily based on the number of days per year that temperature or precipitation observations were above or below percentile thresholds. Station level indices were gridded to produce North American area-averaged time series. The results indicated that the increase in the number of days exceeding the 90th percentile is about the same magnitude as the decrease in the number of days below the 10th percentile. Analysis of extremes farther out on the tails of the distribution (e.g., 95th and 97.5th percentiles) reveals changes very similar to the 90th and 10th percentiles. Annual extreme lowest temperatures are warming faster than annual extreme highest temperatures when the index assessed is the actual temperature, but cold and hot extremes are changing about the same when examined on a normalized basis. On the basis of several measures, heavy precipitation has been increasing over the last half century, and the average amount of precipitation falling on days with precipitation has also been increasing. These observed changes since the late 1960s, decrease in cold extremes, increases in warm extremes, and increases in heavy precipitation, are consistent with a warming planet.


Map from Dai 2012:

a, Percentage changes from 1980–1999 to 2080–2099 in the multimodel ensemble mean soil-moisture content in the top 10 cm layer (broadly similar for the whole soil layer) simulated by 11 CMIP5 models under the RCP4.5 emissions scenario. Stippling indicates at least 82% (9 out of 11) of the models agree on the sign of change. b, Mean sc_PDSI_pm averaged over 2090–2099 computed using the 14-model ensemble mean climate (including surface air temperature, precipitation, wind speed, specific humidity and net radiation) from the CMIP5 simulations under the RCP4.5 scenario. A sc_PDSI_pm value of −3.0 or below indicates severe to extreme droughts for the present climate, but its quantitative interpretation for future values in b may require modification.

Comments:

  1. For those who are not sure what is going on read this.

    I don't actually have a problem with that. I think it is clear that 'normal climate' has shifted in recent years, and will continue to shift as we dump more GHGs into the atmosphere.

    My objection was limited to calling the recent extreme heat-waves the new normal. I don't think they are, not yet. Though such extreme events have become significantly more common.


Leave a Reply

Your email address will not be published.