Global dimming
Dr Jim Haywood
Aerosol Research Manager
- What is global dimming?
- What causes global dimming?
- Is global dimming included in climate models?
- Do observations of global dimming agree with climate models?
What is global dimming?
Global dimming is the observed widespread reduction in sunlight at the surface of the Earth. Global dimming shows significant regional variations. The amount of sunlight received at the Earth’s surface has declined at some observing stations, but not at others.
- Continuous dimming has been observed at Hong Kong
- No dimming has been seen at some sites (such as Lerwick, UK)
- Other sites have shown dimming until around 1980, but subsequently show significant brightening (e.g. Uccle, Belgium and Israel).
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European sulphur emissions in Tg Sulphur per year
Figure 1. The reduction in sulphur emissions since 1980 by European countries.
What causes global dimming?
The most likely cause of global dimming is the interaction with sunlight of microscopic aerosol particles from human activities. Aerosols reflect and absorb sunlight, reducing the intensity of sunlight at ground level. Jim also explains about aerosols and climate change.
Some aerosol particles are produced by human activities such as the burning of fossil-fuels and industrial processes. Global dimming can, therefore, be related to increased pollution, while brightening is related to a reduction in pollution. Sulphur is an example of an aerosol-forming pollutant. Sulphur emissions from European countries have fallen from 18 Tg/y in 1980 to 4 Tg/y in 2000 (see Fig. 1). This partly explains the brightening seen at Uccle in Belgium.
Is global dimming included in computer models of the changing climate?
The effects of global dimming (and global brightening) are included in the global climate models that are used to simulate past and future changes in climate. The Met Office Hadley Centre’s computer models include many aerosol species such as aerosols from sulphur dioxide, smoke from deforestation and stubble burning, mineral dust, and soot.
Do observations of global dimming agree with climate models?
Results from our climate models are consistent with observations of global dimming and global brightening. Climate model simulations show regional dimming over many areas from 1950–1980, due to the increased aerosol emissions.
Air quality regulations introduced in many western countries reduced atmospheric aerosol pollution between 1980 and 2000 causing brightening over Europe (see Fig. 2). However, atmospheric aerosol pollution has continued to increase in the developing countries since 1980, causing continued global dimming (see Fig. 3).
Figure 2. Corresponding changes in the sunlight received at the Earth’s surface. Blue = dimming, yellow and red = brightening.
Figure 3. Changes in the amount of aerosols in the atmosphere between 1950 and 1980, and 1980 and 2000. Yellow and red = increase, blue = decrease.
About Dr Jim Haywood PhD BSc(Hon)
- Leads the Aerosol Research Team
- Is a lead author on the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC)
- Is a lead author on the IPCC’s aviation report
- Sits on the Global Energy and Water Cycle Experiment (GEWEX) Radiation Panel
- Wrote his PhD thesis on the impact of soot and sulphate aerosol on climate
- Has worked as a climate modeller for over 13 years
- Has written more than 35 scientific papers
Climate scientists explain about…
Geoengineering: the risks and rewards of tackling climate change
17 June 2010
explained by Dr Olivier Boucher
Climate change and observations 27 May 2010
explained by Dr David Parker
Clarifiying climate change messages 1 Feb 2010
explained by Dr Richard Betts
Global-average
temperature records 2 Dec 2009
explained by Dr Peter Stott
Climate feedbacks: from the known to the unknown 4 Nov 2009
explained by Dr Ben Booth
Global dimming 20 Mar 2009
explained by Dr Jim Haywood.
Aerosols 19 Nov 2008
explained by Dr Jim Haywood.
The future 19 Nov 2008
explained by Dr Vicky Pope.
You might also be interested in…
Convention on Long-range Transboundary Air Pollution — aiming to limit, reduce and prevent air pollution.
The National Emission Ceilings Directive — seeks to reduce emissions of those pollutants to protect the environment and human health.
