Our understanding of the climate and how it can change

Transcription

1 Our understanding of the climate and how it can change SUM4015 spring 2012 first lecture: 1. The Earth and its climate how does it all work? 2. How do we know we re changing the climate? Bjørn H. Samset - Forsker, CICERO b.h.samset@cicero.uio.no kollokvium.no

2 Syllabus: Climate Change: A Multidisciplinary Approach (on ARK.no) Chapters: 1: Introduction 2: Radiation and the Earth s energy balance 3: The elements of climate 6: The natural causes of climate change 7: Human activities 8: Evidence of climate change

3 Motivation: It s getting hotter

4 Motivation: It s getting hotter According to the 2007 Fourth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC), global surface temperature increased 0.74 ± 0.18 C (1.33 ± 0.32 F) during the 20th century. Source: NASA GISS

5 Part 1: The Earth and its climate Key questions: What is weather and what is climate? What keeps the climate stable from year to year? What factors can, over time, alter the climate?

6 Weather vs climate Whether or not it s «cold» when you wait for the bus or «a very warm spring this year» doesn t tell you anything about climate change only about the current weather.

7 Climate variability and «climate change» The temperature varies all the time with position, season, time of day, It s very hard to measure a «global» temperature, so we measure temp. differences «Climate change» is a change in average properties of the weather e.g. a change in global, annual mean sea surface temperature, rainfall, cloudiness, number or strength of hurricanes, etc. Science says: We need to average over at least 15 years to get anything meaningful

8 What is energy and where does it come from? To understand climate change, we can view the earth as an energy problem. Energy is always conserved it cannot be created or destroyed. Heat which we need more of if there is to be «global warming» is just one special kind of energy. Global warming that the sea surface temperature is increasing - means that either energy is flowing from somewhere in the earth system and into the atmosphere or energy is flowing from outside the earth system and into the atmosphere If we understand the energy flow into and around the Earth, then we understand climate change. (But not the weather that s way more complicated.)

9

10 The Earth/Sun system

11 Radiation balance

12 The land (and sea) albedo Albedo = the fraction of sunlight reflected back to space 100% (red) means everything is reflected back

13 The atmosphere

14 The oceans

15 Radiation balance

16 The «greenhouse effect»? CO2: Certain molecules can store energy as vibrations in the «rubber bands» (atomic bindings) that hold the atoms together. + water vapor!

17 The hydrological cycle

18 The carbon cycle

19 Radiation balance

20 Natural climate change and variability Burroughs mentions e.g. Solar variability Orbital variations Volcanoes Changes in the atmosphere and ocean Tidal forces Continental drift Changes in atmospheric composition (natural) Sudden releases of methane from deep oceans or permafrost Asteroid impacts

21 Solar variability Lockwood and Fröhlich Proc. R. Soc There is considerable evidence for solar influence on the Earth s pre-industrial climate and the Sun may well have been a factor in postindustrial climate change in the first half of the last century. Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth s climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures.

22 NB: We live in extremely stable times! Climate change isn t an issue for the world as such it will manage fine. We, however, have created a fragile society under extremely stable conditions. If those change, so will the foundations of our civilization.

23 Atmosphere/ocean: El Nino Southern Oscillaion (ENSO) Hansen, J., R. Ruedy, Mki. Sato, and K. Lo, 2010: Global surface temperature change. Rev. Geophys., 48, RG4004

24 Atmosphere/ocean: North Atlantic Oscillaions (AMO and NAO) FromTracking the Atlantic Multidecadal Oscillation through the last 8,000 years Mads Faurschou Knudsen et Al, Nature Communications 2, Article number: 178 doi: /ncomms1186

25 Volcanoes Hansen, J., R. Ruedy, Mki. Sato, and K. Lo, 2010: Global surface temperature change. Rev. Geophys., 48, RG4004

26 Conclusions for part 1: The climate is complex, but not too complex to understand To understand the climate, first remove all the variability caused by weather, solar cycle, ocean currents, etc. Key questions: What is weather and what is climate? What keeps the climate stable from year to year? What factors can, over time, alter the climate? What remains is mostly an energy problem. Where does energy get lost or gained, where and how is it transported? Any long-term change in energy flow will result in climate change

27 Part 2: How we affect the atmosphere and hence the climate Key questions: How do human activities influence the atmosphere? How do we know how the climate has changed in the past? How can we link our activities to the current observed changes?

28 CO2-emissions are rising year by year G. Peters et al., Nature Climate Change (2011) Does this matter for the climate? Why? How? How much?

29 We re changing the composition of the atmosphere

30 We re changing the composition of the atmosphere IPCC AR4 See e.g. here: how-do-we-know-that-recent-cosub2sub-increases-aredue-to-human-activities-updated/

31 Emissions of greenhouse gases

32 Radiation balance

33 Paleoclimate: Studying the climate experiments that nature has already performed. Do we understand them?

34 Solar variability Lockwood and Fröhlich Proc. R. Soc There is considerable evidence for solar influence on the Earth s pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century. Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth s climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures.

35 Two main factors: The greenhouse effect from (primarily) CO2 The change in reflectance (albedo) from changes in snow and ice cover We (mostly) understand all this! Both qualitatively and quantitavely.

36 so 390 ppm

37 Energy imbalance per 2007 IPCC det internasjonale klimapanelet: Etablert i 1988, består av flere tusen forskere Mandatet er å vurdere og rapportere Den største og mest enige gruppen av forskere i verdens historie!!! Ny rapport i 2013/ /13/ipcc-femte-rapport/

38 Dust and aerosols Sulphate, nitrate, dust, : Cooling, due to sunblockeffect Soot: Warming and cooling, with the warming effect dominating, since soot is black and hence absorbs sunlight

39 Dust and aerosols Sulphate, nitrate, dust, : Cooling, due to sunblockeffect Soot: Warming and cooling, with the warming effect dominating, since soot is black and hence absorbs sunlight

40 Dust and aerosols Sulphate, nitrate, dust, : Cooling, due to sunblockeffect Soot: Warming and cooling, with the warming effect dominating, since soot is black and hence absorbs sunlight

41 Changes in cloud cover

42 Energy imbalance per 2007 What does this mean in terms of temperature change over time?

43 Climate feedbacks The climate impact from a change in e.g. CO2 is not simply the associated change in greenhouse effect. It s how the whole earth system responds to this change.

44 Climate sensitivity Linking a certain amount of CO2 to a certain temperature increase DT s = l x RF If we double the concentration of CO2 in the atmosphere, how much will the temperature change? Alt gir samme konklusjon: [The climate sensitivity] likely to be in the range 2 to 4.5 C with a best estimate of about 3 C, and is very unlikely to be less than 1.5 C. (IPCC AR4) Vet vi hvordan noe påvirker strålingsbalansen, kan vi beregne temperaturendring

45 Teaser for next lecture: Models provide independent link Havtemperatur Lufttemperatur

46 Teaser for next lecture: Models provide independent link Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences. Thomas R. Karl, Susan J. Hassol, Christopher D. Miller, and William L. Murray, editors, A Report by the Climate Change Science Program and the Subcommittee on Global Change Research, Washington, DC.

47 Human-induced climate change in 5 steps 1. We are changing the composition of the atmosphere, by emissions of long-lived greenhouse gases such as CO2 and methane, and aerosols and other short-lived components Key item: The Keeling curve of CO2 concentration 2. We know from basic science how such gases and particles affect the surface temperature in particular the warming effect of CO2 Key item: The research of Joseph Fourier ( ), John Tyndall ( ), Svante Arrhenius ( ) and many others 3. The global surface temperature of the Earth is indeed increasing Key item: Global temperature analyses by NASA, the UK Met Office and others 4. Sophisticated mathematical models allow us to study causal mechanisms in the Earths climate, using past changes as tests Key item: NASAs James Hansen used a model to predict the temperature effects of the 1991 Mount Pinatubo eruption 5. Only when known human emissions are included do the models reproduce the observed temperature and climate changes Key item: The four IPCC reports and the one coming in 2013 Inspiration: «The Hockey Stick and the Climate Wars», Michael E. Mann, Columbia University Press, 2012

48 Conclusions for part 2 Through various emissions we re changing the energy balance of the Earth Past changes and basic science both tell us how much our emissions will matter in the long run Climate models pull it all together Key questions: How do human activities influence the atmosphere? How do we know how the climate has changed in the past? How can we link our activities to the current observed changes?

49 From the article Communicating the Science of Climate Change, by Richard C. J. Somerville and Susan Joy Hassol, October 2011 issue of Physics Today NB: Be aware of «climate science language»

50

51

52

53

54