>Understanding Scientific Terms About Climate Change

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Certainty vs. Uncertainty

Union of Concerned Scientists – http://www.ucsusa.org
Last Revised: 03/17/10

Uncertainty is ubiquitous in our daily lives. We are uncertain about where to go to college, when and if to get married, who will play in the World Series, and so on.

To most of us, uncertainty means not knowing. To scientists, however, uncertainty is how well something is known. And, therein lies an important difference, especially when trying to understand what is known about climate change.

In science, there’s no such thing as absolute certainty. But, research reduces uncertainty. In many cases, theories have been tested and analyzed and examined so thoroughly that their chance of being wrong is infinitesimal. Other times, uncertainties linger despite lengthy research. In those cases, scientists make it their job to explain how well something is known. When gaps in knowledge exist, scientists qualify the evidence to ensure others don’t form conclusions that go beyond what is known.

Even though it may seem counterintuitive, scientists like to point out the level of uncertainty. Why? Because they want to be as transparent as possible and it shows how well certain phenomena are understood. Scientists have even developed their own phrasing regarding uncertainty, such as “very high confidence” (9 out of 10 chances of being correct) about a certain fact and “very likely” (90 chances out of 100) to describe the chance of an outcome.

Decision makers in our society use scientific input all the time. But they could make a critically wrong choice if the unknowns aren’t taken into account. For instance, city planners could build a levee too low or not evacuate enough coastal communities along an expected landfall zone of a hurricane if uncertainty is understated. For these reasons, uncertainty plays a key role in informing public policy.

However, this culture of transparency has caused problems for climate change science. Climate change deniers link certainty projections with not knowing anything. The truth is, science knows much about climate change. We have learned, for example, that the burning of fossil fuels and the clearing or burning of land creates carbon dioxide (CO2), which is released into the atmosphere. There is no uncertainty about this. We have learned that carbon dioxide and other greenhouse gases build up in the atmosphere and trap heat through the greenhouse effect.

Again, there is no uncertainty about this. Earth is warming, and scientists are very certain that humans are the main reason for the world’s temperature increase in the past 50 years.

Scientists know with very high confidence, or even greater certainty, that:

  • Human-induced warming influences physical and biological systems throughout the world
  • Sea levels are rising
  • Glaciers and permafrost are shrinking
  • Oceans are becoming more acidic
  • Ranges of plants and animals are shifting

Scientists are uncertain, however, about how much global warming will occur in the future (between 2.1 degrees and 11 degrees Fahrenheit by 2100). They are also uncertain how soon the sea ice habitat where the ringed seal lives will disappear. Curiously, much of this uncertainty has to do with—are you ready?—humans. The choices we make in the next decade, or so, to reduce emissions of heat-trapping gasses could prevent catastrophic climate change.

So, what’s the bottom line? Science has learned much about climate change. Science tells us what is more or less likely to be true. The latest climate science underscores that there’s an urgent need to reduce heat-trapping emissions. And that is certain.

Table: Language to describe confidence about facts and the likelihood of an outcome.  SOURCE: IPCC WGI (2007).

Terminology for describing confidence about facts
Very High confidence At least 9 out of 10 chance of being correct
High confidence About 8 out of 10 chance
Medium confidence About 5 out of 10 chance
Low confidence About 2 out of 10 chance
Very low confidence Less than 1 out of 10 chance

Terminology for describing likelihood of an outcome
Virtually certain More than 99 chances out of 100
Extremely likely More than 95 chances out of 100
Very likely More than 90 chances out of 100
Likely More than 65 chances out of 100
More likely than not More than 50 chances out of 100