Significant Figures by Peter Gleick

The latest in a long series of science summaries on climate change from the Intergovernmental Panel on Climate Change (IPCC) has just been released. While the report has a massive amount of information in it, related to a wide range of geophysical implications of climate change, here are some of the key water-related findings for precipitation, evaporation, glaciers, ice mass, and more. While many other findings are reported that have hydrologic implications (such as all the findings related to temperature and warming), I have not usually included them here. Definitions of the confidence of the findings are provided below. The full IPCC summary can be found here:

Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.

There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased.

The frequency or intensity of heavy precipitation events has likely increased in North America and Europe. In other continents, confidence in changes in heavy precipitation events is at most medium.

It is very likely that regions of high salinity where evaporation dominates have become more saline, while regions of low salinity where precipitation dominates have become fresher since the 1950s. These regional trends in ocean salinity provide indirect evidence that evaporation and precipitation over the oceans have changed (medium confidence).

Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent (high confidence)

The average rate of ice loss from the Greenland ice sheet has very likely substantially increased … over the period 1992–2001 to … over the period 2002–2011.

The average rate of ice loss from the Antarctic ice sheet has likely increased … over the period 1992–2001 to … over the period 2002–2011.

There is very high confidence that the extent of Northern Hemisphere snow cover has decreased since the mid-20th century.

There has been some improvement in the simulation of continental-scale patterns of precipitation since the [previous IPCC report]. At regional scales, precipitation is not simulated as well, and the assessment is hampered by observational uncertainties.

Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes. This evidence for human influence has grown since [the previous IPCC report]. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.

It is likely that anthropogenic influences have affected the global water cycle since 1960. Anthropogenic influences have contributed to observed increases in atmospheric moisture content in the atmosphere (medium confidence), to global-scale changes in precipitation patterns over land (medium confidence), to intensification of heavy precipitation over land regions where data are sufficient (medium confidence), and to changes in surface and subsurface ocean salinity (very likely).

It is likely that there has been an anthropogenic contribution to observed reductions in Northern Hemisphere spring snow cover since 1970.

Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent by the end of this century, as global mean surface temperature increases.

Globally, it is likely that the area encompassed by monsoon systems will increase over the 21st century. While monsoon winds are likely to weaken, monsoon precipitation is likely to intensify due to the increase in atmospheric moisture. Monsoon onset dates are likely to become earlier or not to change much. Monsoon retreat dates will likely be delayed, resulting in lengthening of the monsoon season in many regions.

By the end of the 21st century, the global glacier volume, excluding glaciers on the periphery of Antarctica, is projected to decrease by 15 to 55% for RCP2.6 [the low emissions scenario], and by 35 to 85% for RCP8.5 [the high emissions scenario](medium confidence).

The area of Northern Hemisphere spring snow cover is projected to decrease by 7% for RCP2.6 and by 25% in RCP8.5 by the end of the 21st century for the model average (medium confidence).

Finally, it is worth noting this IPCC conclusion:

Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.

[Note: In this Summary for Policymakers, the following terms have been used to indicate the assessed likelihood of an outcome or a result: virtually certain 99–100% probability, very likely 90–100%, likely 66–100%, about as likely as not 33–66%, unlikely 0–33%, very unlikely 0–10%, exceptionally unlikely 0–1%. Additional terms (extremely likely: 95–100%, more likely than not >50–100%, and extremely unlikely 0–5%) may also be used when appropriate. ]


  1. [...] Climate discusses the report here, and Joe Romm has something here. Also, Peter Gleick asks “What does the IPCC say about water?” Oh, and Mark Hertsgaard notes that “Bill McKibben should feel vindicated today.” [...]

  2. #2 Rich Persoff
    Watsonville, CA
    September 30, 2013

    The Tragedy Of The Commons: What belongs to everybody is the responsibility of nobody. No society will sacrifice its short-term interests (survival and gratification of its leaders and supporters) for the long-term good.

    That human-kind will continue to people a depauperate word is probable, given our ingenuity and will to survive.

    However, considering that every human problem is made worse the more people there are, count on Gaia to come up with some “interesting” solutions of her own.

  3. [...] Blog honorable mentions:  Wow, so many today!  Here are even more:  Environmental science students rise to storytelling challenge from the California Water Blog, Guinness: An Irish Water Rights Story from the Hydrowonk blog, and Peter Gleick answers the question, What does the 2013 IPCC Summary Say About Water? [...]

  4. #4 John Russell (Twitter: @JohnRussell40)
    Devon, UK
    October 1, 2013

    Peter; I know you have followed the scientists’ convention of using the ‘likely’/'very likely’/'extremely likely’, etc., indication of statistical confidence in this blog. However I note in the IPCC ‘Headlines’ document [ ] they have in places expressed levels of confidence in a different way. For instance, instead of saying, “there is high confidence snow cover has continued to decrease in extent”; they actually write “snow cover has continued to decrease in extent (high confidence)” . Now I’m a writer not a scientist, so I don’t know whether the two formats are scientifically equivalent, but in terms of understanding by a lay public, the second format is, psychologically speaking, much stronger and less vulnerable to deliberate misrepresentation. Also, as it presents the level of confidence in parenthesis, it’s more clearly identifiable as a scientific term and is less likely to be read as an admission of ignorance — which is how certain people portray it.

    If it’s feasible from a scientific perspective I would recommend the latter format (with levels of certainty in parenthesis) is used from now on when communicating climate science.

    Just a thought.

    • #5 Peter Gleick
      October 1, 2013

      Thanks for the comment. Here, I’ve just laid out the original language (with the description/definitions used by the IPCC). I agree that they could be clearer and more consistent in their communications. Sigh.

  5. [...] The latest in a long series of science summaries on climate change from the Intergovernmental Panel on Climate Change (IPCC) has just been released.  [...]

  6. [...] Peter Gleick drills down the IPCC panel report's findings on water:  What does the IPCC say about water?  Peter Gleick runs down some of the key water-related findings for precipitation, evaporation, glaciers, ice mass, and more here:  What does the 2013 IPCC Summary Say About Water? [...]

  7. [...] And the big ice hat our planet wears is falling off.”  Peter Gleick collates a number of excerpts related to water on Significant Figures, which say that there are likely more regions getting more rain than less; [...]