27 September, 2013

The IPCC summary for policymakers is out. What now?

Sérgio Abranches

The release of IPCC’s Summary Report for Policymakers today has ended speculations that animated the social media over the last few weeks, but has not eliminated controversies. A full view of the scientists’ take on the scientific state of the art on the physics of climate change will have to wait for the final draft of the Working Group I contribution to the IPCC Fifth Assessment Report due on 30 September.

Key points on IPCC’s presentation of its summary report today:

Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. Growing confidence on human causes.

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.

Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. In the Northern Hemisphere, 1983–2012 was likely the warmest 30-year period of the last 1400 years.

Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010. It is virtually certain that the upper ocean warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971.

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.

The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia.Cumulative emissions of CO2  largely determine global mean surface warming by the late 21st century and beyond. Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped. This represents a substantial multi-century climate change commitment created by past, present and future emissions of CO2.

Thomas Stocker, co-Chair of IPCC’s  Working Group I responding to a question about the overnight line by line discussions over the draft said that there were “no fundamental change”, and that the scientists were able to  keep the 18 points they wanted to make and all figures. He also said that over 90% of coastal areas are vulnerable to high sea level events as well as extreme high sea levels events.

The report also stresses that climate models have been significantly improved since the last report, in 2007, and measurement has become more accurate. Instrument bias has been eliminated, particularly on instruments measuring sea level rise.

There were at least two noticeable differences between the draft leaked before the meeting and the official reported released today.

The first one regards the so-called temperature hiatus.

The leaked draft version was:

“Global mean surface temperature trends exhibit substantial decadal variability, despite the robust multi-decadal warming since 1901. The rate of warming over the past 15 years (1998-2012; 0.05 [-0.05 to + 0.15] ºC per decade) is smaller than the trend since 1951 (1951-2012; 0.12 [0.08 to 0.14] ºC per decade).”

Now the final report states that:

“In addition to robust multi-decadal warming, global mean surface temperature exhibits substantial decadal and interannual variability. Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to +0.15] °C per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade).”

This means that the IPCC considers that the importance of this hiatus has been overestimated, and most of it is due to the strong effects of El Niño, in 1998. Thomas Stocker stressed that anyway the last decade was the warmest of all decades in the period 1951-2012. Stocker also stressed the unusually strong El Niño in 1998, as well as the fact that scientists consider the 30-year long trend far more accurate as a basis for predictions.

The report also changed the explanation for the slowdown that led to this hiatus. The draft said that

“models do not generally reproduce the observed reduction in surface warming trend over the last 10-15 years”.

The report includes a much  longer commentary, saying that

“the observed reduction in surface warming trend over the period 1998–2012 as compared to the period 1951–2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend. There is medium confidence that internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of internal variability. There may also be a contribution from forcing inadequacies and, in some models, an overestimate of the response to increasing greenhouse gas and other anthropogenic forcing (dominated by the effects of aerosols).”

In other words the slowdown has been caused in roughly equal parts by natural climate variability, and by a temporary reduction of solar radiation due to volcanic eruptions. Heat absorption by deeper ocean waters may also have contributed to the natural variability side of the explanation. Stocker has mentioned volcanic activity as a noticeable factor during the press conference.

The message for policy-makers is also unequivocal. We are way out of track to meet the political commitment to keep warming below the 2 oC. Governments have failed so far to keep their own commitments to mitigate climate change as well as to fulfill their obligation to reduce collective risk. IPCC’s vice-chairman, Jean-Pascal van Ypersele, said that we are beyond the worst-case scenario, but this is only an indication that we must act now to curb emissions and get back into the track for the best-case scenario, while it is still possible.

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