Variability and Trends in Global Drought. Greg McCabe, USGS, Denver, CO Dave Wolock, USGS, Lawrence, KS

Transcription

1 Variability and Trends in Global Drought Greg McCabe, USGS, Denver, CO Dave Wolock, USGS, Lawrence, KS

2 a. mean global temperature b. Mean global precipitation Global temperature has increased. A warmer climate, with its increased climate variability, will increase the risk of both floods and droughts. (IPCC, 2007) Global precipitation also has increased. Has global warming caused increased occurrence of drought globally?

3 a. mean global temperature b. Mean global precipitation Global temperature has increased. A warmer climate, with its increased climate variability, will increase the risk of both floods and droughts. (IPCC, 2007) Global precipitation also has increased. Has global warming caused increased occurrence of drought globally?

4 Objectives The objectives of this paper are to answer two basic questions regarding global drought occurrence: 1. Given global warming and associated increases in PET, has the occurrence of drought increased during the past century? 2. Are inter-annual changes in drought occurrence controlled mostly by changes in PET, P, or both equally?

5 Data - monthly precipitation (P), temperature (T), and potential evapotranspiration (PET) (Penman Montieth) data for the land areas of the globe (between 60 o S and 70 o N) from the CRUTS3.10 data set through o by 0.5 o resolution - 57,087 grid cells

6 CRUTS3.10 data set 0.5 o by 0.5 o resolution 57,087 grid cells

7 Methods - computed monthly sums of P PET (PMPE) positive values indicate water surplus and negative values indicate water deficit - monthly values were summed to produce annual values of PMPE negative values of annual PMPE indicate drought

8 Percent of global land area with drought conditions (%drought) Percent There is no long-term trend in %drought (-0.2% per 100 years, non-statistically significant). Given noted increases in global T and PET during the past century, it is surprising that drought occurrence has not increased.

9 To determine the explanation for the lack of a long-term trend in global drought occurrence (%drought), we performed experiments where PMPE was computed in two ways - 1. using monthly PET and long-term mean monthly P (monthly P climatology) - PETvar model 2. Using monthly P and long-term mean monthly PET (monthly PET climatology) - Pvar model used these two methods to compute the annual percentage of global land areas in drought (%drought) allowed us to separate the individual effects of variability in PET and P on %drought

10 Percent of global land area (%drought) with drought conditions Trends Complete model -0.2%/100 years (non-sig) PETvar 1.1%/100 years (p < 0.05) Pvar -2.2%/100 years (p < 0.05) complete PETvar Pvar Increases in global P have substantially reduced the effects of increased PET (resulting from increased T) on %drought.

11 When have the effects of increased PET (and T) on %drought been most noticeable? Departures of the percent (%) of global land areas in drought (%drought) computed using variable PETvar model

12 When have the effects of increased PET (and T) on %drought been most noticeable? Departures of the percent (%) of global land areas in drought (%drought) computed using variable PETvar model The most positive effects of PET on %drought occurred after about The post 1987 period is the focus of remaining analyses.

13 Have the effects of increased PET on %drought been consistent across all latitudes? Mean annual departures of the percent (%) of global land area with drought (%drought) for the 1988 through 2009 period by 5 latitudinal bands complete PETvar Pvar 2

14 Have the effects of increased PET on %drought been consistent across all latitudes? Mean annual departures of the percent (%) of global land area with drought (%drought) for the 1988 through 2009 period by 5 latitudinal bands In the high northern latitudes the effects of PET on %drought appear to have been greater than the effects of precipitation. complete PETvar Pvar 2

15 Mean monthly departures for 5 o latitudinal bands, 1988 through 2009 Departure (mm) Departure ( o C) a. T b. P c. PET d. %drought Departure (mm) Departure (%drought) Departure (mm) Positive departures in T and PET in the high northern latitudes have resulted in positive departures of %drought even though P has increased.

16 Conclusions Global %drought has not changed, even though global T and PET have increased. Globally, increases in annual global P have mitigated the effects of increases in PET (and T) on %drought. Positive contributions of increases in PET on %drought are most noticeable after about During the 1988 through 2009, in the high northern latitudes, increased PET during the late winter/early spring resulted in increased %drought even though P also increased. Two questions: (1) do the long-term positive trends in annual global P and PET indicate an enhanced hydrologic cycle? (2) will continued warming and increased PET eventually exceed increases in P and thus result in increased %drought?

17 G.J. McCabe and D.M. Wolock, 2015, Variability and trends in global drought, Earth and Space Science, /2015EA

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