The first of the latest round of IPCC reports was released today in Stockholm.

Ever since 1990 when the first assessment was released, the science community,on a cycle of roughly every five years or so, goes through a huge collaborative effort to collectively agree on what we know about climate change and express this knowledge in a comprehensive assessment report that is reviewed and vetted by scientists and policymakers before being released to the public. All of this effort is overseen and coordinated by the Intergovernmental Panel on Climate Change, an international scientific body established by the United Nations Environment Programme and the World Meteorological Organization, and so it is referred to as the IPCC reports.

There have been four such assessments already completed: in 1990, 1995, 2001, 2007. As has been the practice, this latest fifth assessment will contain reports from three working groups, each looking at different aspects of the problem. The first of these reports, from Working Group 1, scheduled for release on September 30, “assesses the physical scientific aspects of the climate system and climate change.” Today the IPCC released the accompanying summary for policymakers [pdf].

As many of you have already read (for instance here, here, here or here), the report does not contain any bombshells. The topline message is consistent with what we’ve read in previous assessments: global warming is real, is largely due to human activities, and to avoid potentially dangerous climate change (e.g., an increase of 2 degrees Celsius in global temperatures above preindustrial levels), we as a global community will have to limit additional emissions of carbon dioxide (CO2) and other greenhouse gases, and the window to doing that is rapidly closing.

Here are some highlights of the report that I found interesting:

1. Precipitation change: The panel concluded with “high confidence” that precipitation over the mid-latitude land areas of the Northern Hemisphere has increased since 1951. And they predict that “[t]he contrast in precipitation between wet and dry regions and between wet and dry seasons will increase.”

This is something to take note of: incremental changes in temperature are one thing, but changes in precipitation (as we have learned from the recent wildfires and floods out west) can be life-threatening.

2. The “hiatus”: The rise in global temperatures seems to have slowed since the late 1990s; some even say the warming has stopped. The panel’s take on this — which is largely supported by recent papers such as “Distinctive climate signals in reanalysis of global ocean heat content” byMagdalenaBalmaseda et al., “World ocean heat content and thermosteric sea level change (0–2000m), 1955–2010″by Sydney Levitus et al., “Retrospective prediction of the global warming slowdown in the past decade” byVirginieGuemaset al., and “A review of global ocean temperature observations: Implications for ocean heat content estimates and climate change” byJ.P. Abraham et al. —is that the warming has continuedbut has gone mostly into the ocean:

“The observed reduction in surface warming trend over the period 1998–2012 as compared tothe period 1951–2012, is due in roughly equal measure to a reduced trend in radiative forcingand a cooling contribution from internal variability, which includes a possible redistribution ofheat within the ocean (medium confidence).“

You’ll note the authors’ use of “internal variability” as the cause of the “redistribution of heat within the ocean.” Simple translation for “internal variability”: we don’t yet know what’s causing it. Nevertheless, since we’ve had similar hiatuses in the 20thcentury that ended with renewed warming (see for example the period in the ‘50s and ‘60s), there is every reason to believe that the current hiatus is also temporary and the warming trend will soon pick up steam (if you will pardon the pun).

It’s also important that the working group recognizes the existence of a warming slowdown but not a warming stoppage. For example, the current decade is the warmest in the modern record.

Observed globally averaged combined land and ocean surface temperature (1850-2012)

While the rise in global temperatures has seemingly slowed or even stopped since the late 1990s, the oceans have been heating up, meaning that the warming has continued despite a cold Sun and an economic slowdown that slowed greenhouse gas emissions. Summary for Policymakers, IPCC, 2013

3. Climate sensitivity: Much has been written in TheGreenGrok and elsewhere about the climate sensitivity — the amount the equilibrium temperature increases for a doubling of CO2. The reason: it is a useful metric for understanding how sensitive the climate is to the current and future emissions of greenhouse gas we continue to spew into the atmosphere. The previous IPCC report stated that sensitivity as ranging from 2 to 4.5 degrees Celsius. In response to a number of recent papers that concluded the climate sensitivity is lower (see here and here), the new report puts the range from 1.5 to 4.5 degrees Celsius, moving the lower limit to that sensitivity downward.

This is a significant change. Does it mean the climate skeptics were right all along? Hardly. CO2 still warms the atmosphere. And even if we get lucky and the lower limit proves correct, we still don’t get to emit carbon dioxide without limit. It would simply give us more time —perhaps a few decades —to rein in greenhouse gas emissions before things get dicey. And by the way, I do hope that proves to be the case.

4. Attribution of warming to human influences —from likely to very likely to extremely likely: The IPCC reports are consensus documents and therefore tend to err on the conservative side on their conclusions —that’s generally the best way to get universal buy-in. And so it is interesting that with each IPCC assessment, the consensus on the role of humans in driving climate change has grown stronger.

Beginning with the third assessment (AR3), the working group called out human activities as the primary cause of climate change*:

“most of the warming observed over the last 50 years is attributable to human activities.”

But by using the word “likely,” they were hedging their bets, indicating a significant degree of uncertainty.

The fourth assessment (AR4) on the issue was more definitivewith its use of the term “very likely”:

“Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

Now, in the most recent assessment, the group has upped the ante stating:

“Human influence has been detected in warming of the atmosphere and the ocean, in changes inthe 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 AR4. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”

5.Irreversible warming:The group paints a pretty stark picture of how far into the future people will have to contend with our generation’s greenhouse gas emissions:

“A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial time scale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions. Due to the long time scales of heat transfer from the ocean surface to depth, ocean warming will continue for centuries. Sea level rise beyond 2100 ‘is virtually certain.’”

6. Adaptation will be required:The projections for having justa greater than 50 percent chance of limiting warming to less than the 2 degree Celsius threshold —the threshold the international community has identified to avoid dangerous climate change [pdf]—“will require cumulative CO2 emissions from all anthropogenic sources to stay between … 0 and about 1210 [gigatons of carbon] GtC.”

That may sound like a lot of carbon emissions, but we have already emitted about half of this total, at current rates we could easily use up the remaining by mid-century, and the longer we wait the more expensive it’s going to get.

With the numbers seen this way, the odds of keeping warming below 2 degrees Celsius seem slim. The second report of AR5, from Working Group II due out in March 2014 on adaptation, should address some of the challenges we will face in trying to deal with that new normal.

All aboard the train for AR6?

Finally, it is interesting to note that the AR5 report of Working Group 1was prepared by 259 authors from 39 countrieswho responded to 54,677 comments from reviewers.** And that’s just the report from Working Group 1. We all —meaning scientists, policymakers and even the public —depend upon and benefit from the IPCC assessments. I, for example, have used them as the basic source material in courses I’ve taught on the climate. But we’re also talking about a huge expenditure of energy and time by the scientific community. A process that requires hundreds of scientists to take time away from pursuing their research interests to gather information and carry out analyses required for the assessment. And even as the fifthassessment is being released, the IPCC is almost certainly beginning to gather resources and get scientists to commit to the next assessment —AR6.

Before the AR6 train leaves the station, perhaps we should be asking ourselves: Is it worth it? Or, with five assessments under our belt, have the costs of producing these comprehensive reports begun to outweigh the benefits?

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End Notes

* In the second assessment, the working group wrote [pdf]:

“Global mean surface temperature has increased by between about 0.3 and 0.6C since the late 19th century, a change that is unlikely to be entirely natural in origin. The balance of evidence … suggests a discernible human influence on global climate.”

** And they all deserve our thanks and gratitude —the work was all donepro bono.