This graph by the Berkeley Earth Project shows that data from four reputable organisations agree about the rise in the earth’s average surface temperature over the last two hundred years. There really aren’t any grounds for disputing the fact that global warming is happening.

We know it’s happening, and the clear scientific consensus is that it’s caused by the rising carbon emissions and other greenhouse gases we’ve created -plus soot – through industrialisation and globalisation based on burning fossil fuels.  Some key scientific questions about climate change include:

• how much is global warming likely to increase?
• what level of global warming is dangerous?

Where to find the scientists’ answers?

shared from the National Stem Centre website

Climate models have to solve equations for many places – on the Earth’s surface, in the deep oceans and in the high atmosphere. If the models run a few days into the future, this gives a weather forecast.

To predict climate change, the models have to run decades into the future. This needs vast amounts of computer power.

If you want to use your own computer to help in climate modelling, you can sign up to Oxford University’s climateprediction.net project.

What’s causing climate change?
The Fourth IPPC Assessment, in 2007, says that there is widely accepted evidence for increased levels of human-caused (anthropogenic) greenhouse gas (GHG) emissions in the atmosphere, and that these are “very likely” to be the cause of  ” most of the observed increase in global average observed temperatures since the mid-20th century”.

How much is global warming likely to increase?

The 2007 IPPC Assessment estimates that:

•  a rise in average global temperature of between 1.8 degrees Celsius and 4 degrees Celsius is likely by 2100, as a result of a continuing increase in carbon emissions and other greenhouse gases (GHGs)
• this is likely to cause sea levels to rise by between 18 and 59 centimetres by 2100.

The range of the temperature and sea level estimates is down to the fact that they’re based on different future scenarios about demographic, economic and technological forces that drive climate change. In other words, the amount of impact human beings have on the climate depends on:

• population size and make up,
• the state of the economy (in a recession, less economic activity means less fossil fuels are burnt)
• the technology we use – is it more or less energy efficient, more or less energy intensive, more or less based on renewables or on fossil fuels?

However, the data in the 2007 Fourth Assessment Report is now outdated, particularly given this summer’s record-breaking Arctic sea ice melt.

This 2011 video explains some causes of recent changes in sea levels.

The range of estimated temperature rises is also due to varying estimates of climate sensitivity – ie, how much the Earth’s average near-surface temperature would eventually rise if the concentration of GHGs doubled, compared to the pre-industrial levels around 200 or so years ago.

The consensus judgement of the 2007 IPPC Assessment scientists was that the climate sensitivity value was probably between 2 degrees Celsius and 4 degrees Celsius – so, if the concentration of of GHGs doubles compared to pre-industrial levels, the earth’s average temperature is likely to rise by between 2 and 4 degrees Celsius.

What level of global warming is dangerous?

Climate change is already making survival difficult for people in some parts of the world, and a 2009 report from the Global Humanitarian Forum calculated that climate change was already killing 300,000 people a year. The calculations are estimates based on figures  provided by the World Bank, the World Health organisation, the UN, the Potsdam Insitute For Climate Impact Research, and others, including leading insurance companies and Oxfam. A Guardian article about the report  says it  ”was reviewed by 10 of the world’s leading experts incluing Rajendra Pachauri, head of the UN’s Intergovernmental Panel On Climate Change, Jeffrey Sachs, of Columbia University and Margareta Wahlström, assistant UN secretary general for disaster risk reduction.“

The Report identifies clear issues of environmental injustice, with countries in the global south bearing the brunt of climate change. These countries were the home of nearly 98% of the people seriously affected and 99% of all people dying from weather-related disasters. They bore 90% of the total economic losses from climate change. The report says that the populations most at risk are in sub-Saharan Africa, the Middle East, south Asia and the small island states of the Pacific. Britain is among the 12 countries considered least at risk. All but one are industrially developed.

In November 2011, the IPCC published a report that confirmed that there is a direct relationship between climate change and an increase in extreme weather events.

Because of the complexities of all the inter-relating elements of the climate, and because there’s still so much climate scientists don’t know about how the climate works, it’s hard to say with certainty what degree of climate change is dangerous. But putting a figure on the rise in carbon emissions or in global average temperature that is considered dangerous may not be so important. Particularly since the current rise in temperature is already causing extreme weather events that are killing thousands. Recognising we need to do something about it, regardless of the numbers, is what matters.

Numbers may be uncertain, but climate damage is clear

As an example of the uncertainty about numbers, people often quote the figure that average global temperature mustn’t increase by any more than 2 degrees Celsius above the pre-industrial level around two hundred years ago. The Council of the European Union set this target in 1996, on the grounds that if global warming exceeds 2 degrees Celsius, “..irreversible catastrophic events may occur”. (1)

But Richard Betts, Head of Climate Impacts at the UK Met Office, a Lead Author for the 2007 IPCC 4th Assessment Report (Working Group 1) and a Contributing Author for the 2007 IPCC 4th Assessment Report (Working Group 2), recently stated, “Most climate scientists do not subscribe to the 2 degrees “Dangerous Climate Change” meme (I know I don’t).” He points out that, “the relationship between any particular level of global mean temperature rise and impacts on society are fraught with uncertainties, including the nature of regional climate responses…While really bad things may happen at 2 degrees, they may very well not happen either – especially in the short term….We just don’t know.“

Environmental scientist Dana Nuccitelli and Distinguished Professor of Meteorology at Penn State University, Michael E. Mann, conclude,

“Given that it will take a significant effort to avoid doubling the amount of carbon dioxide in the atmosphere, from a policy perspective arguments about the precise climate sensitivity are somewhat irrelevant. Even at the lower end of the estimated sensitivity range, the projected impacts of climate change are likely to be devastating to human civilisation and our environment. What it will take to avoid such a scenario is what we … ought to be focusing on.”

Uncertainties, high stakes, urgent public decisions – what kind of science is this?

This all goes to show that climate science deals with uncertainty, high stakes and urgent public decisions. This kind of science is called  post-normal science. And it’s going to affect how we live. If we, the public, are to have any kind of democratic control over it, I guess we need to get our heads round it.

Sources:

1) ‘Questions and Answers on the Commission Communication, Limiting Global Climate Change to 2 Degrees C’, Memo/07/17, cited in Barrett, Scott, Climate Treaties and the Imperative of Inforcement (p71), in Helm, Dieter & Hepburn, Cameron (eds) (2009), The Economics and Politics of Climate Change. Oxford, Oxford University Press.

Updated 4 April 2013 with link to this paper.

Updated 12 April 2013 with links to this article.