How much warming can we expect?

 

The Earth’s climate system

The Earth’s climate system is complex. It contains oceans, rivers, clouds, ecosystems and ice caps.

(Source: Climate change: the scientific basis for concern, MCV factsheet, 2008; PDF 1.3 MB)

 

The climate system redistributes energy from the sun and creates ‘feedback mechanisms’ that can either amplify or reduce warming at the surface.

By burning fossil fuels, we are rapidly releasing carbon back into the atmosphere—carbon that has taken millennia to become locked away below the Earth’s surface.

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Feedback mechanisms – an example

Higher surface temperatures lead to more evaporation.

More evaporation in turn leads to more water vapour in the atmosphere.

Water vapour is a strong greenhouse gas that adds to the warming at the surface. But more water vapour also leads to more cloud cover and brighter clouds.

More cloud cover and brighter clouds reduce surface warming by reflecting sunlight.

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Climate models

To find out how much warming we can expect, scientists use computer simulations of the climate system. These climate models are programmed to cater for scenarios of increasing greenhouse gases.

Global climate models are multi-dimensional mathematical models that simulate the motions of the atmosphere and oceans for decades, or even centuries.

The global average temperature is projected to increase by 1.1–6.4°C over the 21st century. This projection is from climate models that simulate the future climate using realistic future emissions scenarios (IPCC Fourth Assessment Report).

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Why the broad range in global warming projections?

Two main sources of uncertainty account for the broad range:

We do not know precisely how greenhouse gas concentrations will vary in the future. Due to demographic, economic and technological factors, a range of greenhouse gas emission pathways are possible for the 21st century.

Climate models can predict large-scale trends but they cannot predict the exact state of the Earth’s climate at a point in time. This is especially true the further we look into the future. While all models predict that increasing greenhouse gases will lead to a warmer climate, the amount of warming differs from model to model.

 

(Source: Climate change: the scientific basis for concern, MCV factsheet, 2008; PDF 1.3 MB)

B1= Economic globalisation and realistic action taken to limit CO2 emissions
A2 = Reduced/fractured economic globalisation and no coherent action to limit CO2 emissions
A1B = Limited action on climate change

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Uncertainties in climate models

The complexity and chaotic nature of the global climate system means that models must simplify certain aspects of the climate system, particularly those that occur at small scales. This creates uncertainties in climate model projections.

These uncertainties are well understood and scientists are confident that the models provide reliable insight into how the real climate system works and how it will respond to increased greenhouse gases.

Scientists rigorously assess the ability of models to accurately match the known patterns of present and past climates. Current models are able to reproduce the large-scale patterns and processes of the climate.

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