How do I decipher the odds in climate science?

Climate science, like most professions, has its own jargon. The globe’s climate scientists write very clearly and precisely, if you are familiar with the language of earth science.

Sometimes a little translation is useful.  Today, in Bern, Switzerland, the Intergovernmental Panel on Climate Change released the first of a series of reports that together comprise the Fifth Assessment Report. This is a big deal and has been in the works for three years.

The report highlights to many, many findings and represents the consensus from among 600 authors in 32 countries who relied on 9,200 scientific publications backed up by 54,677 comments from 1089 expert reviewers from 55 countries.  The report has been reviewed this week line by line and approved by up to 195 countries.

(If you are interested in reading more about how this international consensus process works, see this link to a very snappy description “The Dance of Mice and Elephants.” Disclosure: my publisher thinks it worthy enough they blogged it today to coincide with the above 2013 report release. The whole chapter is online for free.)

But two findings reported today stand out as unequivocal and significant.  First, humans are responsible for the rise in global warming today. And, second, the warming trends are bad and getting worse far faster than we can reverse. About the best we can do now is to try to cap the future warming we have already baked into the atmosphere from getting worse.

Over the course of 23 years and five assessment reports, the IPCC has made the following statements about whether the data show the Earth is warming and whether human activities are part of the warming:

The Progression of Confidence by the IPCC in Its Findings

•  1990: “Earth has been warming, and continued warming is likely.” (First Assessment Report)
•  1995: “Balance of evidence suggests discernible human influence.” (Second Assessment Report)
•  2001: “Most of warming of past 50 years [is] likely (odds 2 out of 3) due to human activities.” (Third Assessment Report)
•  2007: “Most of warming [is] very likely (odds 9 out of 10) due to greenhouse gases.” (Fourth Assessment Report)
• 2013: “It is extremely likely (>99% probability) that human influence has been the dominant cause of the observed warming since the mid-20th century.” (Fifth Assessment report)

Source: See www.ipcc.ch/ipccreports/assessments-reports.htm.

Let’s take a look at some of the direct quotes from today press conference in Bern. We’ll offer a translation, where helpful.

“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.”

Very little translation needed here: “unprecedented over decades to millennia” simply means the warming measured now has no precedent going back thousands of years.

“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 (medium confidence).”

Note the italicized words “likely” and “medium confidence”: these are statements of probability and confidence in that probability. In this case, because weather records are not as precise the further back we go (thermometers were not invented until the 17th century), we can’t be 100% sure the past 30 years are absolutely the warmest since 600 AD. 

As many scientific reports do, this report, contains many such “likelihood” and “confidence degree” statements. So let’s review the terminology to understand what scientists mean by them.

Throughout the Climate Change 2013 report, the authors assign a likelihood to events that are occurring now or are projected to occur. The phrases on the left below reflect the probabilities listed in the middle column. Sample events with some of these corresponding likelihoods are listed on the right.

Table 2 Some Likelihood Definitions and Examples

Terminology                 Likelihood of the occurrence/outcome               

Virtually certain = >99% probability of occurrence                 

Examples:

•     Less frequent and warmer cold days and nights over most land areas;
•     More frequent and warmer hot days and nights over most land areas.
•     Increased insect outbreaks

Very likely                  90 to 99% probability

Examples:

•     If atmospheric CO2 level stabilizes at double today’s level, average global temperature will rise by 1.5 degrees Celsius.
•     The frequency of heavy precipitation events will increase.
•     The frequency of warm spells or heat waves will increase over most land areas.                               

Likely                  66 to 90% probability

Examples:

•     If atmospheric CO2 level stabilizes at double today’s level, average global temperature will rise by between 2 and 4.5 degrees Celsius.
•     Areas affected by drought will increase.
•     Intense tropical cyclone activity will increase.
•     Extreme high sea-level events will increase.

The authors of IPCC reports assign confidence levels to the major statements based on their assessment of the current knowledge on that topic. Each impact is often accompanied by the authors’ confidence level in brackets. This level of confidence is Very High, High, Medium, Low or Very Low.

Some Confidence Degree Definitions and Examples

Terminology  = Degree of confidence in being correct

Very high confidence = At least 9 out of 10 chance of being correct                 

•     At two or more degrees warmer, floods, drought, and erosion will increase and water-quality will decrease.
•     Sea-level rise will increase the salinization of groundwater, decreasing freshwater in coastal regions.

High confidence = About 8 out of 10 chance

•     A warming of up to two degrees will increase the frequency of forest or grassland fires and their intensity, especially in areas that suffer from drought.
•     Most coral reefs will bleach and die-off.

Medium to high confidence = About 6  or 7 out of 0 chance

•     An increase of up to two degrees of warming will produce an increase in Category 4 to Category 5 tropical cyclone storms.  In addition, their impact on coastal regions will be exacerbated by sea-level rise.
•     Malnutrition, infectious and diarrheal diseases, malaria, and direct fatal accidental injuries from flood, heat, and drought will increase.

Medium confidence = About 5 out of 10 chance                 

•     For a warming between 1 and 3 degrees, some areas in low latitudes will experience a decrease in productivity for some cereals, which may be offset by an increase in productivity in the middle to high latitudes.
•     Current effects on human health are small, but discernible.

Low confidence = About 2 out of 10 chance

Very low confidence = Less than a 1 out of 10 chance

As grim as the findings are above, they are from the prior 2007 IPCC report, as I have not yet had time comb through today’s full report.  

But let’s read a few more lines from today’s initial report’s Summary for Policymakers. Notice the likelihood and confidence levels where indicated are quite explicitly stated.

“Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system.”

For the non-scientist, “positive radiative forcing” means a self-reinforcing cycle of energy flow, e.g. the warmer the ice caps get, the faster they melt and the more open water results that absorbs more heat that in turn helps melt more ice.

“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 AR4 (the last report in 2007). It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”

No translation needed!

“The atmospheric concentrations of carbon dioxide (CO2), methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years. CO2 concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land use change emissions. The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification.”

The industrial age is generally considered to have begun in 1750 as coal mining and burning coal proliferated, initially in Europe. By 1850, burning coal was a world-wide phenomenon that continues today. The term “anthropogenic” means “human-caused.”

“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).”

Since we live in the Northern Hemisphere, we can expect less and less snow here in New York, especially along the Atlantic seaboard.

“The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia (high confidence). Over the period 1901–2010, global mean sea level rose by 0.19 [0.17 to 0.21] m.”

We can expect our Hudson River to gain in level over the rest of this young century.

Sorry, no fancy pictures in this column, just some facts from science.