Climate changes in Finland, but how? Jouni Räisänen Department of Physics, University of Helsinki

Transcription

1 Climate changes in Finland, but how? Jouni Räisänen Department of Physics, University of Helsinki

2 Outline Some basic questions and answers about climate change How are projections of climate change constructed? How is the global climate expected to change? What about Finland?

3 Some basic questions and answers about climate change 3

4 Why is climate expected to change? Because the external factors that regulate climate are expected to change Greenhouse gas concentrations (CO 2, CH 4, etc.) Aerosol concentrations? Solar output, volcanic eruptions?? However, internal dynamics of the climate system (atmosphere, oceans, land, cryosphere) also induce climate variability may either amplify or suppress forced climate changes may be stronger or weaker than forced climate change (depending on what aspect of climate is considered, time horizon, spatial scale etc.)

5 Why is the world expected to get warmer? Because increases in greenhouse gas concentrations (particularly CO 2 ) are expected to dominate over other changes in external conditions internal climate variability The magnitude of the change will likely be amplified by positive feedback effects due to increased atmospheric water vapour reduced ice and snow cover

6 Why is future climate difficult to predict in detail? Uncertainties in external forcing magnitude of future greenhouse gas emissions how much of the emissions will stay in the atmosphere etc. Feedback effects within the climate system changes in cloudiness changes in atmospheric and oceanic circulation etc. Internal climate variability not a big issue for century-scale projections of global warming a major uncertainty for some aspects of regional climate change (particularly in the short term, when other uncertainties are still relatively small)

7 How are projections of climate change constructed? 7

8 How are forecasts (or projections ) of climate change constructed? Greenhouse gas and aerosol emissions Assumptions about population, economy, technology etc. Greenhouse gas and aerosol concentrations Carbon cycle model etc. Climate change Climate model Each step of the calculation has its own sources of error.

9 Scenarios: CO 2 emissions and concentrations Emissions (10 9 t / year) Concentration (ppm) Growth of population and economy vs. development of cleaner technology? To stop the increase in CO 2 concentration, larger emission reductions are required than assumed in any of these six SRES scenarios

10 Construction of climate change projections (ii) External factors affecting climate (e.g., greenhouse gas concentrations) as a function of time Atmos. Land Ocean Ice Climate model: laws of nature as a computer program, as well as the current knowledge allows Long weather forecast Climate = statistical properties of weather

11 Global climate model (traditional set-up) Atmosphere Land surf. Ocean Sea ice Exchange of heat ( ), water ( ) and momentum ( ) between the model components Other components (carbon cycle, interactive vegetation, atmospheric chemistry...) are included in many of the most recent models.

12 Structure of an atmospheric model Global 3-dimensional grid (D ~ D ~ 2 ~ 200 km; ~30-50 levels) Primitive equations time derivatives of temperature, wind, pressure, humidity etc. in each grid point Time integration (Time step ~ 30 min, for 100+ years New time derivatives in each step.) 30 min years

13 Primitive equations x F fv x p u y u v x u u t u F y fu y p v y v v x v u t v p c p Q p c RT p T y T v x T u t T p RT p 0 p y v x u S q p q y q v x q u t q S X p X y X v x X u t X The same equations as used in weather forecast models Nearly exact description of atmospheric physics (in principle) Difficulty: sub-grid scale source terms: F x, F y, Q, S q Water vapour

14 Observed vs. simulated changes in global mean T, (IPCC 2007) observed simulated observed simulated Sun + volcanoes + greenhouse gases + aerosols (58 simulations by 14 models) Sun + volcanoes (19 simulations by 5 models)

15 How is the global climate expected to change? 15

16 Temperature change (ºC) Projected change in global mean temperature, from 1990 to B1 A1T B2 A1B A2 A1FI Small emissions Uncertainties taken into account (i) climate models (ii) carbon cycle For comparison - Global mean warming in 20 th century ~0.7ºC Difference in global mean temperature between the last glacial maximum (20 ky BP) and present: 4-7ºC Large emissions

17 Annual mean temperature change, in the next 100 years? C - mean over 21 climate models - medium-size greenhouse gas emissions (A1B scenario)

18 Annual mean temperature change, normalized by its global mean C/ C - The warming in nearly all land areas is likely to exceed its global mean value

19 Changes in annual precipitation (%), in the next 100 years? Tendency for most wet areas to get wetter and for most dry areas to become drier

20 What about Finland (and nearby areas)? 20

21 Changes in temperature and precipitation in Europe, in the next 100 years? Winter (Dec-Feb) Summer (Jun-Aug) Annual mean - mean over 21 climate models - medium-size greenhouse gas emissions (A1B scenario) - 4ºC in annual mean T: Kemi Helsinki, Helsinki Berlin

22 The changes vary from model to model Change in annual mean temperature in the next 100 years, in 21 climate models

23 What about changes in ocean circulation? The Atlantic meridional overturning circulation (popularly misnomed as the Gulf Stream) weakens in most model simulations of 21st century climate. In some models, this leads to cooling in the northern North Atlantic, but the cooling is unlikely to extend to Europe (if the models are right!).

24 Changes in annual mean temperature and precipitation in Southern Finland (60ºN, 25ºE) Good qualitative agreement, although substantial quantitative uncertainty. NB: these simulations used the same emissions scenario (A1B). The emission uncertainty is not included. Temperature change (ºC)

25 Global mean warming vs. warming in southern Finland (60 N, 25 E) 1/21 18/21 2/21 Models with large global warming also typically simulate large warming in Finland, but the correlation is not perfect

26 Climate in Helsinki in this century? Annual mean T ( C) Annual precipitation (mm) Assumptions underlying the forecast (1) Linear trends of 4ºC / 100 yr in temperature and 15% / 100 yr in precipitation ( the best estimate from models) (2) Interannual variability exactly as in the 20 th century (in reality, the details of the variability cannot be predicted)

27 What about snow? More precipitation in winter more snowfall (if winter precipitation still as snow) more snow on ground (if it does not melt away)? or Less snow on ground, because in a milder climate - A larger fraction of precipitation is rain - Thaws melt snow faster and more often

28 Sodankylä Jokioinen Winter climate (September-June), averaged over 11 regional climate model simulations Temperature Total precipitation Snowfall Snow amount (water equivalent)

29 Sodankylä Jokioinen Winter climate (September-June), averaged over 11 regional climate model simulations Temperature Total precipitation Snowfall Snow amount (water equivalent)

30 Sodankylä Jokioinen Winter climate (September-June), averaged over 11 regional climate model simulations Temperature Total precipitation Snowfall Snow amount (water equivalent)

31 Sodankylä Jokioinen Winter climate (September-June), averaged over 11 regional climate model simulations Temperature Total precipitation Snowfall Snow amount (water equivalent)

32 Interannual variability of snow amount in Southern Finland ( Vantaa ) in three regional climate models Bars = maximum monthly mean snow water equivalent in each winter Blue = snow amount exceeds the mean for Red = snow amount less than half of the mean for Yellow = %

33 Conclusions Increases in CO 2 and other greenhouse gases will make the world warmer during this century (but we know at best approximately how much warmer!) The most probable aspects of climate change in Finland will include warming at a rate of 1-2 times the global mean increase in precipitation particularly in winter decrease in snow and ice cover Natural climate variability will continue, occasionally amplifying, occasionally counteracting the anthropogenic changes

34 Many more details (in Finnish): ACCLIM final report, 2009