Contemporary climatic changes in North America and Northern Eurasia with foci on extreme events and transitions through environmentally and socio-economically significant thresholds

Contemporary climate models send several very different messages regarding changes in the energy and water cycle over northern extratropical land areas that are leading to climate extremes of different kinds: “disproportional rates of warming”, “when it rains it pours”, “summer dryness”, “intensification of forest fires in the boreal zone”, and others. All of these terms/slogans characterize several (although frequently correlated among themselves) changes in occurrences of extreme events. We test the validity of these statements for the regions of Northern Eurasia and North America with a dense network of long-term time series of daily observations by quantifying several lines of evidence of contemporary changes that have lead to changes in the frequency (and intensity) of extreme events in high- and mid-latitudes of the Northern Hemisphere land areas. Among these extreme events are (a) very heavy rainfall events, (b) prolonged no-rain intervals, (c) indices that characterize severity of the “fire” weather, and (c) timing and magnitude of peak streamflow.

Special attention is paid to recent climatic changes in North America and Northern Eurasia with a focus on characteristics of the seasonal cycle such as temperature transitions through environmentally and socio-economically significant thresholds (e.g., no-frost period, duration and “strength” of growing season and cold seasons, frequency and intensity of hot and cold spells) and energy accumulated indices that are proportional to the societal need to cope with seasonal weather (e.g., heating-degree and cooling degree days). These thresholds do not necessarily characterize extreme events, but rather changes in their dates, duration, totals, or distribution within the year which may affect society. For example, in the United States, we found that during the past century a difference between the durations of vegetation and no-frost periods systematically decreased, thus reducing the probability of “false spring” events over most of the country. However, in the important agricultural region of the southeastern US, this difference went up by 7 days per century causing an increase of the chances of an unfortunate impact of late frosts on crops. Our analyses for North America show increasing rates of changes in most of characteristics of the temperature seasonal cycle during the past few decades. Some of these changes can be considered as positive while others cause concern. In particular, in the area of the North American Monsoon (southwestern US) we observe strong warming that together with the precipitation deficit increases chances of detrimental weather conditions such as extremely hot nights that affect human health, prolonged no-rain periods, and higher values of the fire weather indices. Generally, the impact of hot nights on human health (a relative frequency of heart attacks) is well established. Now, with minimum temperature continuing to rise, this impact has become more severe everywhere in the eastern and southern United States. During the past four decades, there was an approximately 60% nationwide increase in the number of “hot nights” (with Tmin >75°F or > 23.9°C) and in the densely populated northwestern U.S. the number of such nights (usually very low before the 1960s) more than doubled.

The occurrences (a) of days with unusually high convective available potential energy (CAPE) and/or (b) of prolonged periods with CAPE close to zero do not automatically mean an occurrence of “extreme events”. However, these characteristics may serve as necessary conditions for such events and their changes may provide clues about changes in the frequency and magnitude of intense precipitation events and droughts. We analyzed tails of CAPE and low troposphere humidity distribution, persistence and systematic changes (trends) in occurrence of their anomalously high/low values during several past decades using the North American Regional Reanalysis. The corroboration of these changes with changes in precipitation extremes will be discussed at the Conference.