by Judith Curry
The IPCC has just published the Summary for Policymakers from the forthcoming Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX).
Well, it is a bit difficult to judge this based only on the Summary for Policy Makers, but I know this literature pretty well, so I will jump in and provide an assessment of their assessment.
Things I like
The character and severity of impacts from climate extremes depend not only on the extremes themselves but also on exposure and vulnerability. In this report, adverse impacts are considered disasters when they produce widespread damage and cause severe alterations in the normal functioning of communities or societies. Climate extremes, exposure, and vulnerability are influenced by a wide range of factors, including anthropogenic climate change, natural climate variability, and socioeconomic development.
JC comment: finally, recognition of things other than AGW.
To accurately convey the degree of certainty in key findings, the report relies on the consistent use of calibrated uncertainty language, introduced in Box SPM.2.
JC comment: many of the conclusions characterize confidence by specifying both dimensions (e.g. high agreement, medium evidence). This provides a little bit more information than what the IPCC has previously done.
Climate Change: A change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings, or to persistent anthropogenic changes in the composition of the atmosphere or in land use.
[INSERT FOOTNOTE 2: This definition differs from that in the United Nations Framework Convention on Climate Change (UNFCCC), where climate change is defined as: “a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.” The UNFCCC thus makes a distinction between climate change attributable to human activities altering the atmospheric composition, and climate variability attributable to natural causes.]
JC comment: finally, a useful definition of climate change
A changing climate leads to changes in the frequency, intensity, spatial extent, duration, and timing of extreme weather and climate events, and can result in unprecedented extreme weather and climate events. Changes in extremes can be linked to changes in the mean, variance or shape of probability distributions, or all of these (Figure SPM.3). Some climate extremes (e.g., droughts) may be the result of an accumulation of weather or climate events that are not extreme when considered independently. Many extreme weather and climate events continue to be the result of natural climate variability. Natural variability will be an important factor in shaping future extremes in addition to the effect of anthropogenic changes in climate.
Trends in exposure and vulnerability are major drivers of changes in disaster risk (high confidence). Understanding the multi-faceted nature of both exposure and vulnerability is a prerequisite for determining how weather and climate events contribute to the occurrence of disasters, and for designing and implementing effective adaptation and disaster risk management strategies. Vulnerability reduction is a core common element of adaptation and disaster risk management.
JC comment: the issue of vulnerability was pretty much glossed over in IPCC AR4 WGII, and this seems to be the key issue.
Projected precipitation and temperature changes imply possible changes in floods, although overall there is low confidence in projections of changes in fluvial floods. Confidence is low due to limited evidence and because the causes of regional changes are complex, although there are exceptions to this statement. There is medium confidence (based on physical reasoning) that projected increases in heavy rainfall would contribute to increases in local flooding, in some catchments or regions.
JC comment: this seems reasonable to me (unlike most of the other projections, see below).
There is low confidence in projections of changes in large-scale patterns of natural climate variability. Confidence is low in projections of changes in monsoons (rainfall, circulation) because there is little consensus in climate models regarding the sign of future change in the monsoons. Model projections of changes in El Niño – Southern Oscillation variability and the frequency of El Niño episodes are not consistent, and so there is low confidence in projections of changes in this phenomenon.
E. MANAGING CHANGING RISKS OF CLIMATE EXTREMES AND DISASTERS
JC comment: this whole section is pretty good.
Points of concern
There is evidence from observations gathered since 1950 of change in some extremes. Confidence in observed changes in extremes depends on the quality and quantity of data and the availability of studies analyzing these data, which vary across regions and for different extremes. Assigning “low confidence” in observed changes of a specific extreme on regional or global scales neither implies nor excludes the possibility of changes in this extreme. Extreme events are rare which means there are few data available to make assessments regarding changes in their frequency or intensity. The more rare the event the more difficult it is to identify long-term changes. [3.2.1] Global-scale trends in a specific extreme may be either more reliable (e.g., for temperature extremes) or less reliable (e.g., for droughts) than some regional-scale trends, depending on the geographical uniformity of the trends in the specific extreme. The following paragraphs provide further details for specific climate extremes from observations since 1950.
JC comment: 60 years is not sufficient for drawing any confident conclusions related to the impact of natural variability on extreme events.
There is evidence that some extremes have changed as a result of anthropogenic influences, including increases in atmospheric concentrations of greenhouse gases. It is likely that anthropogenic influences have led to warming of extreme daily minimum and maximum temperatures on the global scale. There is medium confidence that anthropogenic influences have contributed to intensification of extreme precipitation on the global scale. It is likely that there has been an anthropogenic influence on increasing extreme coastal high water due to increase in mean sea level. The uncertainties in the historical tropical cyclone records, the incomplete understanding of the physical mechanisms linking tropical cyclone metrics to climate change, and the degree of tropical cyclone variability provide only low confidence for the attribution of any detectable changes in tropical cyclone activity to anthropogenic influences. Attribution of single extreme events to anthropogenic climate change is challenging.
JC comment: The attribution of extreme events to AGW depends on a credible attribution of global climate change to AGW. At best, we have “very likely” of “most” (>50%). How that uncertainty is integrated into extreme event attribution here is not at all clear.
Confidence in projecting changes in the direction and magnitude of climate extremes depends on many factors, including the type of extreme, the region and season, the amount and quality of observational data, the level of understanding of the underlying processes, and the reliability of their simulation in models. Projected changes in climate extremes under different emissions scenarios5 generally do not strongly diverge in the coming two to three decades, but these signals are relatively small compared to natural climate variability over this time frame. Even the sign of projected changes in some climate extremes over this time frame is uncertain. For projected changes by the end of the 21st century, either model uncertainty or uncertainties associated with emissions scenarios used becomes dominant, depending on the extreme. Low-probability high-impact changes associated with the crossing of poorly understood climate thresholds cannot be excluded, given the transient and complex nature of the climate.
JC comment: This is fine, except they forgot about natural variability, and the fact that climate models don’t do a very good job with this.
There is high confidence that changes in heat waves, glacial retreat and/or permafrost degradation will affect high mountain phenomena such as slope instabilities, movements of mass, and glacial lake outburst floods. There is also high confidence that changes in heavy precipitation will affect landslides in some regions.
JC comment: this statement does not account for uncertainties in future projections of heat waves, glacial retreat, and permafrost degradation.
Extreme events will have greater impacts on sectors with closer links to climate, such as water, agriculture and food security, forestry, health, and tourism. For example, while it is not currently possible to reliably project specific changes at the catchment scale, there is high confidence that changes in climate have the potential to seriously affect water management systems. However, climate change is in many instances only one of the drivers of future changes, and is not necessarily the most important driver at the local scale.
JC comment. What a mangled statement. What is the point of a statement like this: There is high confidence that changes in climate have the potential to seriously affect water management systems.
Things I don’t like
Overconfidence in future projections:
Models project substantial warming in temperature extremes by the end of the 21st century. It is virtually certain that increases in the frequency and magnitude of warm daily temperature extremes and decreases in cold extremes will occur in the 21st century on the global scale. It is very likely that the length, frequency and/or intensity of warm spells, or heat waves, will increase over most land areas. Based on the A1B and A2 emissions scenarios, a 1-in-20 year hottest day is likely to become a 1-in-2 year event by the end of the 21st century in most regions, except in the high latitudes of the Northern Hemisphere, where it is likely to become a 1-in-5 year event (See Figure SPM 3A). Under the B1 scenario, a 1-in-20 year event would likely become a 1-in-5 year event (and a 1-in-10 year event in Northern Hemisphere high latitudes). The 1-in-20 year extreme daily maximum temperature (i.e., a value that was exceeded on average only once during the period 1981–2000) will likely increase by about 1°C to 3°C by mid-21st century and by about 2°C to 5°C by late-21st century, depending on the region and emissions scenario (based on the B1, A1B and A2 scenarios).
It is likely that the frequency of heavy precipitation or the proportion of total rainfall from heavy falls will increase in the 21st century over many areas of the globe. This is particularly the case in the high latitudes and tropical regions, and in winter in the northern mid-latitudes. Heavy rainfalls associated with tropical cyclones are likely to increase with continued warming. There is medium confidence that, in some regions, increases in heavy precipitation will occur despite projected decreases of total precipitation in those regions. Based on a range of emissions scenarios (B1, A1B, A2), a 1-in-20 year annual maximum daily precipitation amount is likely to become a 1-in-5 to 1-in-15 year event by the end of the 21st century in many regions, and in most regions the higher emissions scenarios (A1B and A2) lead to a stronger projected decrease in return period.
It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged.
JC comment: this relies on climate models, which are worth a grain of salt when it comes to tropical cyclones (see my previous post)
It is very likely that mean sea level rise will contribute to upward trends in extreme coastal high water levels in the future. There is high confidence that locations currently experiencing adverse impacts such as coastal erosion and inundation will continue to do so in the future due to increasing sea levels, all other contributing factors being equal. The very likely contribution of mean sea level rise to increased extreme coastal high water levels, coupled with the likely increase in tropical cyclone maximum wind speed, is a specific issue for tropical small island states.
JC comment: this ignores the other factors that determine local sea level change, which may very well dominate a predicted mean sea level rise (whose future magnitude is highly uncertain).
This report is better than I expected, although I suspect that some of their conclusions are based on weak arguments (we will have to wait for the full report). The two most important aspects IMO are the recognition of the importance of natural variability and also vulnerability. The dominance of natural variability for the past 40-60 years in determining extreme events makes the AGW extreme events attribution exercises (see here) seem even more pointless. The weakest part of the report is the high confidence level of the future projections (including one “virtually certain.”) I suspect that different authors worked on the “Observations” section than those working on the “Future” chapter; too bad the “Future” authors didn’t read the “Observations” section first.