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Robustness of Anthropogenically Forced Decadal Precipitation Changes Projected for the 21st Century

March 20th, 2018

Key Findings

  • We use large suites of climate model simulations to assess whether projected shifts in decadal-mean precipitation over the next several decades, occurring in response to increases in greenhouse gases and changing aerosols, can be distinguished from natural climate variability on a regional scale.
  • In these large sets of model simulations, we find that projected shifts in decadal-mean precipitation were distinguishable from natural variability over more than 1/3 of the globe for the period 2000-2009 (relative to 1950-1999).
  • The earliest distinguishable signals include a moistening over high latitudes and the tropics and a drying over eastern subtropical oceans. Among the earliest distinguishable shifts over the US are a moistening over the northeastern US and a drying over the southwestern US.
  • In response to commonly used estimates of future changes in greenhouse gases and aerosols, shifts in decadal-mean precipitation are distinguishable from natural variability over approximately 70% of the globe by 2050 and close to 90% by 2100.

Honghai Zhang and Thomas L. Delworth. Nature Communications. DOI: 10.1038/s41467-018-03611-3

Precipitation is characterized by substantial natural variability, including on regional and decadal scales. This relatively large variability poses a grand challenge in assessing the significance of anthropogenically forced precipitation changes. The authors use multiple large ensembles of climate change experiments to evaluate whether, on regional scales, anthropogenic changes in decadal precipitation mean state (i.e., ensemble average) are distinguishable – i.e., outside the range expected from natural variability. Relative to the 1950-1999 period, simulated anthropogenic shifts in precipitation mean state for the 2000-2009 period are already distinguishable over 36-41% of the globe—primarily in high latitudes, eastern subtropical oceans and the tropics. Anthropogenic forcing in future medium-to-high emission scenarios is projected to cause distinguishable shifts over 68-75% of the globe by 2050 and 86-88% by 2100. Anthropogenic shifts in decadal precipitation mean state will exceed the bounds of natural variability over most of the planet within several decades.

Decadal variability in precipitation can have drastic impacts on environment and society. For example, the notorious 1930s Dust Bowl, a persistent decade-long drought, is one of the most devastating environmental catastrophes that have stricken the United States during the past century. Projecting future decadal changes in precipitation—particularly those caused by anthropogenic activities—is critical to inform policies on water resource management, agricultural development, infrastructure planning, food security, urban development, and national security, and more. Decadal changes in precipitation are dominated by internal climate variability that is inherently unpredictable beyond a decade (owing to the chaotic nature of the climate system). This dominance of unpredictable internal climate variability (noise) presents an enormous challenge in the projection and assessment of anthropogenically caused decadal changes in precipitation (signal) owing to the weak signal to noise ratio.

The authors used a large set of simulations (about 21,000 years in total) from 2 climate models, the National Center for Atmospheric Research Community Earth System Model version 1 (CESM1) and the GFDL Forecast-oriented Low Ocean Resolution (FLOR) flux-adjusted model to assess the robustness of anthropogenically forced precipitation changes. Despite the dominant role of internal climate variability on regional and decadal time scales, shifts in precipitation mean state caused by anthropogenic forcing can still be distinguished from natural variability. These results show distinguishable anthropogenic shifts in precipitation mean state over the majority of the planet by the middle of the current century. On regional scales, the anthropogenically forced decadal shifts in precipitation mean state are becoming progressively more distinguishable from natural climate variability with each decade over more areas of the globe.

Fraction (%) of global area with distinguishable changes in annual precipitation mean state as a function of decades in GFDL FLOR (blue, 2000-2050) and NCAR CESM1 (red, 2000-2100).