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Global Warming The heat is on! What do you know about global warming? • Did you know: – the earth on average has warmed up? – some places have got colder in the past 250 years? – Temperature, rainfall, storms, drought, sea level, sea ice and glaciers are all changing due to global warming? – We think that human activities are partly responsible. Proof: The Earth is getting warmer And the northern hemisphere is probably the warmest it’s been for at least 1000 years… Mann et al., Science 1999 (Northern Hemisphere only) Global average temperature is one way of measuring the climate of the Earth What is climate? • “Global warming” is a popular term for climate change • Climate is the average weather – “ Climate is what we expect, weather is what we get” • Reading climate? • Climate is easier to predict than weather! To CD! What affects the climate system? What can affect climate? • Climate is a balance between the Earth being heated by the Sun, and cooling by sending out heat itself Changing the solar radiation that we get •Clouds •Particles like dust •Changes to snow and ice •Volcanoes Changing the infrared radiation the Earth sends to space •“Greenhouse” gases • Clouds THE GREENHOUSE EFFECT: identified in 1896! ..most escapes to outer space and cools the earth... SUN …but some IR is trapped by some gases in the air, thus reducing the cooling…. Sunlight passes through the atmosphere.. ..and warms the earth. Arrhenius 1896 Infra-red radiation is given off by the earth... “Human” Global warming • Without some natural greenhouse effect the Earth would be at -15 degrees C (Brr….) • We get this from the natural amounts of water vapour and carbon dioxide in the atmosphere • But human activities add carbon dioxide and other greenhouse gases to the atmosphere • More greenhouse effect • “Global warming” More greenhouse gases leads to more trapping of IR and a warmer surface SUN More energy is absorbed by the Earth than it emits CO2 Sunlight passes through the atmosphere.. ..and warms the earth. CO2 CO2 Infra-red radiation is given off by the earth... Arrhenius 1896 Other human effects on climate? • Particles (or aerosols) in the atmosphere reflect sunlight back to space and can cause cooling in local areas (and also gives us air quality problems). But small changes compared to greenhouse effect. • Large scale agriculture changes the Earth’s surface which affects the radiation reflected back Climate lucky dip! The future of climate change? By 2100: • Increase in global mean temperature of 1.4°C - 5.8°C. • Sea level rise by a further 0.09m – 0.88m. Climate changes over the next few decades are predicted to be much larger than we have seen so far, but there is quite a range of “uncertainty” in the predictions… IPCC (2001) Global change • Changing the temperature patterns around the globe • Changing wind and rainfall patterns • Global climate change And in the UK? 2080 temperature change (K) 2080 precipitation change (%) Source: Mat Collins, Hadley Centre Impacts of climate change may be disastrous……. ANNUAL NUMBER OF PEOPLE FLOODED Change from the present day to the 2080s (unmitigated emissions) UK Climate Impacts Programme Water Supply Water availability will be the major issue in the coming century. Pressure on water supplies comes not only from climate change but also human use. Countries shown currently use more than 20% of water resources Food supplies Percentage change in average crop yields for staple crops: wheat, maize and rice. Yellow, brown and red areas denote lower yields But note increased yields in some northern latitudes due to more favourable climate How do we predict climate change? • We only have one real Earth to experiment with • so.. We build artificial ones… inside a computer This involves equations that describe things like • How the air moves • How the ocean currents work • How water freezes and melts and evaporates and condenses (forming clouds!) • We have to divide the earth up into squares and calculate these things only every few 100kms The Met Office The earth is represented by a grid of squares, typically of length 250 km, and by a stack of layers. This gives us a 3-D picture of the circulation of the atmosphere and oceans Probability forecasts? • We need to know what is most likely to happen, not just what can theoretically happen! High Likelihood Low Temperature change To CD Summary • Climate change is real. • Global warming is most likely to happen, • • although different parts of the globe will warm by different amounts At least over the past 50 years, human activities have contributed to climate change Future changes will be larger than those we have seen already To CD