SAGES CESD

Anthropogenic CO2 and – a historical perspective

Prof. Simon Tett

Aims & Outline CESD

• Try and show how climate science has developed over the last 150 years or so. • How can trace amounts of CO2 influence global climate? • Very much a physical approach. • Modelling and observations • Finish with evidence for human influence on climate • And predictions of future change. Sources:

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• A Vast Machine – Paul N Edwards • The Discovery of Global Warming – Spencer R. Weart • The 4th Assessment Report: The Physical Science Basis. Solomon et al.

Weather vs Climate CESD

• What is climate? • The average weather? • The kind of weather we expect? • What does climate change mean? • Simplest view – global mean temperature. Climate requires a global view CESD

• To see climate change requires the integration of many different observations from across the world. • Meteorologists learnt very early that weather forecasting beyond 2-3 days requires global models. • Climate system moves energy around the globe so to understand it we need to model the entire climate system. Basic Physics: Heat balance CESD

• Temperature of the is set by the balance between incoming and outgoing energy. • At equilibrium outgoing must balance incoming • Outgoing energy depends on temperature. • Joseph Fourier used this to make first calculation of Earth’s temperature. • Concluded it was freezing and that something (the atmosphere) must stop the heat escaping. John Tyndell (1860’s) CESD

Made measurements of infra-red absorption and found that Methane and CO2 where strong absorbers – in fact opaque at the concentrations used. Also found that water vapour absorbed infra- red radiation. http://upload.wikimedia.org/wikipedia/en/e/e9/YoungJohnTyndall_CopiedFromBioOfEdwardFranklandByColinRussell.jpg Svante Arrhenus CESD In 1896 Arrhenus considered feedbacks. What would happen if there were some extra CO2 in the atmosphere? Then the Earth would warm. The atmosphere would hold more water vapour. Water vapour is a greenhouse gas and so would cause the surface temperature to increase which in turn… He computed that if CO2 were halved the planet would cool by about 5K. The first attempt, which we recognize, to compute . http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1903/arrhenius-bio.html Observational Perspective CESD Guy Callendar CESD In 1938 argument was that CO2 and water absorbed at similar places in the spectrum. Callendar using more recent results showed that this was not the case so CO2 would affect climate. He then synthesised observational data to see if any evidence of change.

From http://www.rmets.org/pdf/qjcallender38.pdf Observing Climate Change CESD • Observing system not stable • Climate changes slowly compared to observing system.

Correcting for Homogenisation CESD

• Various approaches: – Compare with near-neighbours. – Build models (buckets) – Run the new and old observing systems in parallel for a few years. Global Mean Temperature – a more

CESD recent picture

From Brohan et al, 2006 Recent spatial trends CESD The Carbon Cycle CESD Hans Seuss CESD Realised that fossil fuels contained no 14C. So by looking at century old wood and modern wood could make an estimate of the fossil fuel input to the atmosphere. Early measurements suggested ocean taking up most C. Gilbert Plass CESD Using early computers to show that in the upper atmosphere CO2 absorption bands had gaps so upper atmosphere wasn’t opaque in the infrared. Implies that changes in CO2 concentration would change the Planetary energy balance and thus the surface temperature. He estimated that human increases in CO2 could increase temperature by about 1K/century.

http://lightbucket.files.wordpress.com/2009/02/gilbertplass.jpg Roger Revelle. CESD That it took the upper ocean about decade to absorb an extra molecule of CO2. **But** that ocean buffering means upper ocean then emits CO2 back to the atmosphere.

Munk W H PNAS 1997;94:8275-8279 ©1997 by National Academy of Sciences Dr Charles Keeling CESD

Successfully measured atmospheric CO2 concentrations in various places. Founded the Mauna Loa record. Which continues today. http://www.esrl.noaa.gov/gmd/obop/mlo/programs/coop/scripps/img/img_scripps_Keeling97.jpg Keeling, 1960 CESD

Increase in atmospheric CO2 at south pole Implications (early 1960’s) CESD

• Changes in CO2 will cause climate warming. • The oceans won’t absorb the CO2 from fossil fuel burning. • Measurements suggest CO2 is increasing. • Humans can influence global climate. • 1965 report mentioned climate change as a possible issue. More recent views CESD

Now at about 390 ppm. In 1960 at about 310 ppm. Airborne fraction about 0.5 mostly being taken up by terrestrial biosphere. Other Greenhouse Gases CESD

• CO2 and water vapour are not the only greenhouse gases. • Key ones are: • Methane • Nitrous Oxide • Ozone • And the CFC’s & HFC’s. Last 650K CESD “Energy” Balance models CESD

• Essentially models of Earth’s surface temperature • Consider energy flows • Parameterise on global scales (albedo, feedbacks and Combining Lapse rate with Radiative

CESD equilibrium

From Fig. 6.9 of Taylor “Elementary Climate Physics”.

In the troposphere the equilibrium temperature computed using radiative equilibrium is unstable. For small perturbations a parcel will rise and be less dense than its surroundings. Thus it will keep rising until it is no longer unstable. Simple conceptual model of the

CESD greenhouse effect

Outgoing longwave radiation to space

Energy in from Sun Simple conceptual model of the

CESD greenhouse effect

Energy in from Sun

Outgoing longwave radiation to space Ht

Temperature What happens if concentration of

CESD Greenhouse gases are reduced?

Energy in from Sun

Outgoing longwave radiation to space Radiation out is the Ht same but it comes from lower in the atmosphere. So surface temperature falls.

Temperature Mikhail Budyko CESD

Budyko build a simple (0-d) model of the Earth’s radiation balance. Essentially a balance between out going radiation and incoming radiation. Key insight was that in a colder world there would be more snow and ice. Snow and ice would increase the planetary albedo and thus cool the climate. Found that the climate system could exhibit Hysteresis. General Circulation Models CESD

Manabe and GFDL CESD

Manabe joined GFDL in the late 1950’s. There he and others built a radiative- convection model of Earth’s atmosphere. Using the idea that relative humidity stays constant, two key processes were radiation and convection. Later he joined forces with Kirk Bryan to build a fully coupled atmosphere/ocean model of the Earth’s climate. Used this to explore many phenomenon. L F Richardson CESD

In 1923 published a book in which he reported an attempt to forecast the weather numerically. Work carried out while Richardson was working as an ambulance driver in the first world war. Climate Modelling CESD

• General Circulation Models (GCM’s) developed from numerical weather prediction models – Take physical laws and apply them to atmospheric and oceanic motions. – Key is that GCM’s are built bottom up. – Interested in “Emergent Phenomenon”, such as statistics of data, rather than detailed evolution. General Circulation Models

CESD 3-D model of the circulation of the atmosphere and ocean

Fundamental equations: • Conservation of momentum • Conservation of mass • Conservation of energy • Equation of state

Parameterized Processes

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•Unresolved motions and processes affect the large scale flow so their effect needs to be parameterized. Slingo From Kevin E. Trenberth, NCAR Radiation CESD Numerical Modelling CESD

Cray Y-MP ~ 1990 L. F. Richardson circa 1920 Since the 1960’s super-computer computational power increased by factor of 16 every decade. Over my career increased 200-300 fold HECToR – 2008 Moore’s Law and Supercomputers CESD

Doubling time of peak supercomputer performance is about 18 months.

Number of transistors doubles every 2 years. But as they get smaller they go faster. Early Met Office experiments CESD

Bushby and Hinds, 1956 using Leo I made first numerical integration at the Met Office. Since early 1990’s Met Office has had unified model – the same atmospheric model for climate and weather http://www.leo-computers.org.uk/images/leo_1_b.jpg forecasting Charney Report CESD

In the summer of 1979 Charney brought together Climate scientists from the US , Sweden, Britain and Australia. Reviewing general circulation model results they concluded that climate sensitivity was between 1.5 and 4.5 K. This result been little modified since then. 2007 IPCC report reported a likely range of 2-4.5 with a best estimate of 3 K. CESD

• In the early 1980’s Crutzen and Burks speculated that a Nuclear war between the USSR and the USA would inject large amounts of smoke into the Stratosphere • There it would have a long lifetime and reflect solar radiation back into space • Cooling the planet with devastating impacts on http://www3.mpch-mainz.mpg.de/~air/crutzen/ Agriculture • “Nuclear Winter” Paul Crutzen • Later calculations suggested more of a “Nuclear Autumn” • Another example of Climate modelling influencing policy. Susan Solomon & Ozone Depletion CESD Concern about Ozone depletion from CFC’s and Nitrogen Oxides peaked in early 1970’s. Atmospheric Chemistry models suggested that CFC concentrations would need to rise to very high levels to cause http://en.wikipedia.org/wiki/File:Susan_Solomon-Desk_With_Globe.jpg significant ozone depletion. Discovery in 1984 by Farnham of enormous ozone depletion over Antarctica suggested that understanding of chemistry. Solomon’s contribution was to detect the chlorine and hypothesise that surface chemistry on polar strat. clouds important. Lead to Montreal Protocol which stopped production of CFC’s. As CFC’s are strong GHG’s reduced climate change. IPCC assessment reports CESD 1st Assessment report – 362 pp, Pub 1990. John Houghton

4th Assessment report – 996 pp, Pub 2007. Susan Solomon 1st Assessment Report (1990) CESD

• Certain: – natural greenhouse effect – Emissions …from human activities are increasing the atmospheric concentrations… • Confident – CO2 been responsible for ½ the enhanced greenhouse effect. – Human emissions have perturbed the natural, balanced, carbon cycle. 1st Assessment Report (1990) CESD

• Predict – Global warming of 0.3 (0.2-0.5) K/decade. – Rise not steady with 1K warming relative to 1990 by 2025 – Sea-level rise of about 6 (3-10) cm/decade mainly due to ocean expansion • Judgement – Global mean surface air temperature increased by 0.3 to 0.6 C over the last century – Size of warming broadly consistent with climate models but also natural variability. 4th Assessment Report (2007) CESD

• CO2 concentrations exceeds anything found in the last 650K years (from ice cores) • Primary source is burning of fossil fuels with some from land use change. • Warming of the climate system is unequivocal • Paleo-climate data suggests that recent warming not occurred in the last 1300 years. • Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. 4th Assessment Report CESD

• Equilibrium climate sensitivity is likely to be in the range 2 to 4.5 K with a best estimate of about 3K, and is very unlikely to be less than 1.5 C • For the next two decades, a warming of about 0.2K/decade is projected. • Continued greenhouse gas emissions … would cause further warming and induce many changes … during the 21st century that would very likely be larger than those observed.

Human Influence on Climate CESD

Globe, Land, Ocean and individual continents all likely show human induced warming. Warming effect of greenhouses gases likely offset by other human and natural drivers Future Predictions CESD

Projections of Future Changes in Climate CESD

Best estimate for low scenario (B1) is 1.8 C (likely range is 1.1 C to 2.9 C), and for high scenario (A1FI) is 4.0 C (likely range is 2.4 C to 6.4 C).

Projections of Future Changes in Climate CESD Projected warming in 21st century expected to be greatest over land and at most high northern latitudes and least over the Southern Ocean and parts of the North Atlantic Ocean

Projections of Future Changes in Climate CESD

Precipitation increases very likely in high latitudes Decreases likely in most subtropical land regions

Summary CESD

• My early 1960’s scientific community had some inkling that CO2 would influence climate. • By late 1970’s consensus was that humans could cause dramatic and rapid changes in climate. • By late 2000’s had evidence that world had warmed due to human activities. • Model predictions suggest future climate change will be dramatic.