What we know and what we don't know
Posted on 12 July 2011 by Verity
This is a cross-post from the Carbon Brief.
“...The science of global warming is far from settled”
Nigel Lawson, “An appeal to reason: a cool look at global warming”, 2008.
Climate skeptics often claim that although increasing levels of carbon dioxide may be causing global temperatures to rise, there are many uncertainties in climate science - with the implication that it would be unwise to put in place potentially expensive or disruptive measures to reduce emissions.
For those not actively working within the field of climate science it can be difficult to determine just how sure scientists are of what they are saying.
In order to do so it is important to understand the method by which science proceeds. Science progresses by testing theories, examining results, repeating the tests and extending the theories. It's this process that allows valid results to be confirmed, and disproven theories to be abandoned. Over time, as a large supporting body of evidence is built and attempts to disprove a theory have failed, that theory can be considered sound. Although we can never be absolutely certain that a theory is true, we can be confident in it given a large volume of supporting evidence and numerous failed attempts to disprove it.
(Well, that's the theory. In practice, the process is usually a bit messier).
So here’s a brief article summarising what bits of climate science we have strong evidence for, and what we are less sure about.
What we have strong evidence for:
Greenhouse gases, including carbon dioxide, warm the planet – the theory that gases including carbon dioxide, methane, nitrous oxide and water vapour retain heat in the atmosphere was first suggested 150 years ago. Svante Arrhenius went on to calculate the potential effect of changing CO2 on climate in 1896. That was followed by over a century of discussion, testing, and refinement of the theory, a process which provided a wealth of supporting evidence.
Other researchers initially argued against the theory, but they were unable to disprove it. The warming effect of carbon dioxide can in fact be demonstrated in the laboratory. As Richard Somerville, climate scientist and distinguished Professor Emeritus at Scripps Institution of Oceanography puts it:
“The greenhouse effect is well understood. It is as real as gravity.”
Atmospheric CO2 is increasing, and has been since the industrial revolution. This is shown by direct measurements from ice-cores and the atmosphere. Levels of the greenhouse gases methane and nitrous oxide have also risen markedly over the same period.
Human activities are causing that rise in CO2. We know that burning fossil fuels and deforestation releases carbon dioxide. Scientists have demonstrated that much of the increase in CO2 in the atmosphere comes as a result of human activities by studying its isotopic composition. CO2 which comes from burning fossil fuels has a distinct manmade signal (explained further here). The proportion of CO2 in the atmosphere that has this “human fingerprint” is rising. The rise in other greenhouse gases such as methane and nitrous oxide can also be attributed to human activities including agriculture and fossil fuel use.
The average global temperature is rising at the surface of the planet (atmospheric and sea surface temperatures). Since it is near impossible to distinguish temperature trends over the short term from ‘noise’ in the data, climate scientists prefer to consider the longer-term temperature trend. This shows an increase of around 0.75˚C since the beginning of the 20th century. A mass of independent evidence - for example the rising temperature of the oceans, rising humidity, rising sea level and melting sea ice - from many different sources also indicate that the world is warming.
Skeptics frequently cite solar activity or other natural climate variation to explain the warming trend. All of these natural elements have been thoroughly investigated by climate scientists, and have been ruled out as the dominant cause of warming.
These fundamental principals, added to the fact that there are no plausible competing theories to explain the observed warming trend, have convinced 97% of active climate scientists that humans are a significant contributing factor to global warming. The IPCC concluded in its most recent report that there is a 90% likelihood that human activities have led to overall warming since pre-industrial times.
What is still under discussion:
What our future emissions will be. Future emissions are likely to depend on several key factors, including global population, energy resource availability, technology, and policy. The IPCC have developed a series of potential greenhouse gas emissions scenarios, based on different global development storylines. These are what are used for the IPCC’s climate projections for the 21st century.
How much temperatures will rise from any given emissions pathway - There are significant uncertainties in assessing how much temperatures will rise because of the difficulties inherent in modelling a system as complex as the climate.
The range of emissions scenarios project that temperatures will increase by between 1.8˚C and around 4˚C by the end of this century. We cannot yet tell which of these emissions scenarios is the most likely to happen - although it has been suggested that our current emissions could put us on track for a temperature rise of 4˚C or more.
We cannot know precisely what the effects of temperature rise will be. Where, when and how severe will the physical impacts be? How will societies respond? Making these kind of predictions is particularly difficult when it comes down to the regional level. When the IPCC assessed the mass of evidence it was found that the impacts of warming are likely to be predominantly negative, particularly at the higher projected temperatures. For example, at a temperature rise of 3 - 4°C above pre-industrial levels we could see:
- Increased water stress for around one sixth of the world’s population
- 20 - 30% of plant and animal species at increased risk of extinction
- Increased coastal damage from floods and storms
- Oceans acidification and coral mortality
- Diminished net carbon uptake by ocean and land sinks
- Decreased cereal production at low latitudes and in some high latitude regions
- Increased injury and mortality from heat waves, floods and droughts - increased burden on health services
Overall, the issue is not "are we having an effect on our planet?" but "what is the scale and timing of the effect we are having on our planet?". Uncertainty in one area of climate science (for example in future projections from climate models) does not mean other areas (for example the evidence that temperature is currently rising) are any less robust.
Thanks to PIRC for the concept and some of the words!