Recent climate variability, trends and the future

Transcription

1 Recent climate variability, trends and the future

2 Summary of observed variability and change

3 While there is no doubt that the global average surface air temperatures has risen over the period of instrumental records (by about 0.8 deg. C), the time series shows considerable variability from year to year and decade to decade. Warming up to about 1940 was followed by slight cooling to about 1970, then by renewed warming. Source:

4 High latitude annual and seasonal temperature anomalies (55-85 N) for evaluated with respect to means. The smoothed line represents results from a nine-point lowpass filter [courtesy of J. Eischeid, Climate Diagnostics Center, Boulder CO]. The annual time series is broadly similar to that for the globe, with warming up to about 1940, followed by cooling to around 1970s, and then renewed warming. The annual time series masks strong seasonal differences. These results are based primarily on land stations, and Arctic data are very sparse in the early part of the record.

5 Surface air temperature trends, , for winter, spring, summer and autumn. Areas in grey have insufficient data for analysis. The recent warming is strongest in high northern latitudes, especially in autumn and winter. This is termed Arctic amplification. Source:

6 Arctic sea ice extent has been accurately monitored from satellites since As assessed over the period of record, there are downward trends in extent for all months, strongest at the end of the melt season in September (11% per decade). September 2007 set a record low, 2008 was second lowest. Both years saw an open Northwest Passage. Sept 14, 2008: 4.51 million sq. km Sept 16, 2007: 4.13 million sq. km

7 Mean sea ice draft from submarine sonar data (September-October) within regions for which co-located measurements are available from early cruises ( ) and cruises in the 1990s. Data are seasonally adjusted to 15 September to account for seasonal variability [courtesy NSIDC, Boulder, CO; based on Rothrock et al., 1999]. These data provided some of the first evidence that the sea ice cover is also thinning.

8 Fields of sea ice age based on an ice age tracking algorithm applied to satellite and buoy data confirm findings from the submarine sonar records. The Arctic Ocean has lost much of its store of old, thick ice, leaving younger and thinner ice that is vulnerable to melting out in summer. Recent thinning is also evident from analyses of sea ice freeboard based on satellite altimetry data

9 With the reduction in summer sea ice extent, winds have a longer fetch over open water, leading to increased wave action, promoting local coastal erosion. These before and after photos are from Shishmarev Island, AK. Courtesy IARC, Dave Sanches

10 NCEP Surface air temperature anomalies for the five-year period minus the period , based on data from the NCEP reanalysis. There is strong warming over the Arctic Ocean in autumn. This reflects loss of sea ice, which allowed for large transfers of heat from the Arctic Ocean to the atmosphere. Temperature anomalies are not strong in summer, as melting ice keeps the surface air temperature close to the freezing point Source: Serreze et al., 2009

11 The recent temperature anomalies are linked to reduced sea ice cover, which allows ocean heat to warm a considerable depth of the atmosphere. Cross section (by height and latitude) of autumn (September- November) temperature anomalies from the NCEP atmospheric reanalysis for the years , relative to NCEP temperature anomalies, minus means, along a transect (left to right) from 50º N to the pole along the date line and from the pole southward to 50º N along the prime meridian. Also shown for the transect are September anomalies in ice concentration and the number of days with ice cover (ice concentration greater than 0.55). The light blue boxes along the zero anomaly line indicate land. (adapted from Serreze et al., 2009)

12 Warming of Atlantic Inflow Moorings at Svinoy and Fram Strait From Polyakov et. al, 2005

13 2007 Melting Day Anomalies The extent of the Greenland ice sheet undergoing surface melt in summer has increased. However, the time series is quite noisy. Melting Index Time Series Marco Tedesco, City College of New York-CUNY, Koni Steffen, Steve Nerem, CIRES

14 Monthly anomalies and 12-month running anomalies of snow cover extent for over: (a) Northern Hemisphere lands, (b) North America and (c) Eurasia [courtesy of D. Robinson, Rutgers University, Piscataway, NJ]. There is no clear trend. The pattern is more of a step function, with the general dominance of negative snow cover anomalies since the 1980s, largely due to spring and summer deficits.

15 Annual and seasonal precipitation anomalies (55-85 N) for evaluated with respect to means. The smoothed line represents results from a ninepoint low-pass filter [courtesy of J. Eischeid, Climate Diagnostics Center, Boulder CO]. While there has been a general increase in northern high latitude precipitation, most of this occurred during the earlier part of the record. This is also when station records were especially sparse!

16 Changes in Annual River Discharge to the Arctic Ocean Upward trend for total discharge,but with large variability, dominated by Eurasia. No change for North American rivers. Source: McClelland et al., GRL, 2006.

17 Net precipitation (P-E) from the ERA-40 reanalysis based on calculation of the vapor flux convergence for the Arctic Ocean and Arctic terrestrial drainage (cold seasons and warm season). There are no obvious trends. This does not square well with the upward trend in river discharge aggregated for the terrestrial drainage. Source: Serreze and Barrett, When combined with results on the next slide, it is clear that we still have some serious problems to address if we are to better diagnose changes in the Arctic hydrologic cycle.

18 Precipitation vs. Runoff Trends over Largest Siberian Rivers (Berezovskaya, Yang & Kane, GRL, 2004) precipitation and runoff anom alies Ob basin 0.2 Yenisei basin 0.2 Lena basin 0 0 NCEP NCEP NCEP UDel -0.2 UDel -0.2 UDel CRU -0.2 CRU -0.2 CRU Runoff -0.2 Runoff -0.2 Runoff yr yr

19 Regional Greening of the Arctic Trends in photosynthetic activity from (GIMMS-G AVHRR Vegetation indices)) Significant positive trends Significant negative trends There is regional greening of the Arctic, but there are negative trends in boreal forest areas. Courtesy Scott Goetz, Woods Hole

20 Changes in Shrub Abundance: Chandler River, AK 1949 Chandler River, 50 miles S. of Umiat: Sturm, Racine and Tape: Fifty Years of Change in Arctic Alaskan Shrub Abundance

21 2001

22 110 o E 130 o E 150o E Permafrost Temperatures are Increasing Alaska: 4 to 6 C increase in 20th Century, 2 to 3 C in the last 30 years Siberia: >3 C increase from mid-1950s to 1990 Canadian Arctic: 1 to 3 C increase in past several decades Tibetan Plateau: up to 1.0 C increase since 1970s 70 o N 60 o N Temperature Departure ( C) Russian Permafrost Temperature 0.2 m; Trend = C/decade 0.4 m; Trend = C/decade 0.8 m; Trend = C/decade 1.6 m; Trend = C/decade 3.2 m; Trend = C/decade Year IPCC 2007

23 The North Atlantic Oscillation and Northern Annular Mode (two names for largely the same thing)

24 The positive (left) and negative (right) modes of the NAO. In the positive mode, both the Icelandic Low (marked L) and the Azores High (marked H) are strong, and the storm track brings heat and moisture into northern Europe and the Arctic via the North Atlantic. In the negative NAO mode, the Icelandic Low and Azores High are both weak, and the storm track is shunted south.

25 Another view of the NAO, showing distributions of (a), (b) winter sea level pressure and (c),(d) sea level pressure anomalies for composites of winter months representing high index and low index states of the NAO, (e) the change in sea level pressure from the high index composite to the low index composite [from Dickson et al., 2000, by permission of AMS]. The strong (weak) Icelandic Low in the positive (negative) phase of the NAO is obvious.

26 Yet another way of looking at the NAO, as the first empirical orthogonal function (EOF) of winter (December-March) sea level pressure for the North Atlantic sector over the period based on Trenberth and Paolino [1980] [from Dickson et al., 2000, by permission of AMS].

27 The Northern Annular Mode (NAM) or Arctic Oscillation (AO) - the NAO s hemispheric scale counterpart, expressed as the Normalized leading EOF of winter (December- March) sea level pressure anomalies over the Northern Hemisphere (20-90 N). The NAM pattern is displayed in term of amplitude (hpa) obtained by regressing the hemispheric sea level pressure anomalies upon the leading principal component time series. The contour interval is 0.5 hpa and the zero contour has been excluded. The data cover the period [from Hurrell et al., 2003, by permission of AGU].

28 Normalized indices of the winter (December-March) NAO, based on the difference in normalized sea level pressure between Lisbon, Portugal and Stykkisholmur/Reykjavik, Island, PC1 of Atlantic sector sea level pressure (20-70 N, 90 W-40 E), and PC1 of Northern Hemisphere sea level pressure (20-90 N). The latter time series describes the NAM. The heavy solid lines represent the indices smoothed to remove fluctuations with periods less than four years [from Hurrell et al., 2003, by permission of AGU]. The obvious conclusion is that the time series of the NAO and NAM (AO) are very similar. Note the upward trend towards the positive phase from about 1980 through the mid 1990s. Over the past decade, the NAO has bounced between positive and negative states.

29 Changes in surface temperature (x10-1 C) corresponding to a unit deviation of the NAO index, computed over the winters (December-March) of Regions of insufficient data are not contoured. [from Hurrell et al., 2003, by permission of AGU]. Conclusion: the NAO/NAM has widespread effects on northern high latitude temperature; in particular, in the positive mode, warm conditions extend across northern Eurasia.

30 Anomalies in winter (December-March) surface air temperature ( o C) for the period with respect to means. Temperature anomalies of > 0.25 C are indicated by dark shading, and those < are indicated by light shading. The contour increment is 0.10 C for negative anomalies and the 0.25, 0.50, 1.0, 1.5 and 2.0 C contours are plotted for positive anomalies. Regions with insufficient temperature data are not contoured [from Hurrell et al., 2003, by permission of AGU]. Note the similarity with the temperature anomaly patterns associated with the NAO/NAM temperature (previous slide). Conclusion: high latitude warmth over the period can be related in part to the generally positive phase of the NAO/NAM.

31 The 30-year ( ) linear trends (January through March) in (left) sea level pressure expressed as 1000 hpa geopotential height and (right) surface air temperature. Total trends are shown in the top panels. The middle panels show the part of the trends linearly congruent with the monthly NAM (AO) index. The bottom panels show the components of the trends that are not linearly congruent with the AO. The contour intervals are 15 m (30 yr)-1 (-22.5, -7.5, ) for sea level pressure and 1 K (30 yr)-1 (-1.5, -0.5, ) for surface air temperature [from Thompson et al., 2000, by permission of AMS]. Conclusion: part, but certainly not all of the observed geopotential height and temperature trends over this period relate to the NAM trend.

32 Positive minus negative NAO difference field of cyclone events for the cold season based on index extremes over the period The contour interval is 20. Positive differences are shown by solid contours with negative differences shown by dash-dot contours [from Serreze et al., 1997, by permission of AMS]. When the NAO is positive (negative) cyclone activity in the North Atlantic is shifted north (south).

33 Vertically-integrated meridional moisture flux crossing 70 N in winter expressed as a function of longitude for composites of winter (DJF) months representing high-index and low-index NAO extremes. The winter mean is also shown [from Dickson et al., 2000, by permission of AMS]. The northward-shifted cyclone track in the positive NAO mode promotes a stronger poleward moisture flux through the North Atlantic gateway

34 The 29-year ( ) linear trends (total % change) in winter precipitation and the component of the trends linearly congruent with the NAM (AO) index. The precipitation database is for land areas only. Contour intervals are 10% (-10, 10, 20...). Dark shading indicates increased precipitation of at least +10%, with light shading indicating reduced precipitation of at least -10%. The zero contour is omitted and negative contours are dashed [from Thompson et al., 2000, by permission of AMS]. Note the dipole in trends between northern Europe (positive) and southern Europe (negative) and (from previous slides) how this links with the NAM trends over this time period and the NAO signal in cyclone activity.

35 The median ice border at the end of April for the periods and , corresponding, respectively, to minimum and maximum phases of the NAO index. The more northerly position of the ice edge for the period corresponds to a reduction in ice extent of about 587,000 km2 from the period [from Dickson et al., 2000, by permission of AMS].

36 Large scale trends in observed winter sea ice motion and summer sea ice concentration (a) and regressions on the prior winter NAM (AO) index (b). Results are based on the period Shading is proportional to the magnitude of the ice concentration trends. [adapted from Rigor et al., 2002, by permission of AMS]. The to panels look very similar to each other. Positive AO winters invoke a counterclockwise anomaly in sea ice motion, pulling ice away from the Eurasian coast and promoting growth of thin ice in these regions. This thin ice then readily melts in summer, leading to negative anomalies in summer ice concentration. The ice circulation pattern associated with the positive AO also works to flush thick ice out of the Arctic Ocean into the North Atlantic via Fram Strait.

37 The summer dipole pattern

38 Recent years, exampled above for 2007 and 2008, have seen a summer dipole pattern, characterized by positive sea level pressure anomalies over the North American side of the Arctic Ocean, and negative anomalies over Eurasia. This has favored warm southerly winds over the Chukchi and East Siberian seas between the pressure cells, favoring summer ice melt. Source: J. Stroeve (NSIDC) and the NOAA Climate Diagnostics Center

39 The 925 hpa temperature anomaly fields associated with the simmer dipole pattern for 2007 and Note the especially strong positive anomalies over the East Siberian Sea in 2007, which led to strong summer ice melt. Source: J. Stroeve (NSIDC) and NOAA Climate Diagnostics Center.

40 Projected changes through the 21 st century

41 Components of global radiative forcing, 2005 relative to A positive forcing equates to a radiation imbalance at the top of the atmosphere, with net solar input exceeding longwave emission to space. This leads to warming. A Source: IPCC negative forcing leads to cooling. Human activities have led to an estimated positive radiative forcing forcing of 1.6 W m -2. Source: IPCC-AR4

42 Putting a radiative forcing of 2 Watts/m 2 in perspective 1 Christmas light per square meter around the entire planet 500 Trillion Christmas lights, on 24 hours a day, 365 days a year 600 x global annual electrical consumption

43 Figure 10.4 The projected global mean annual average temperature change over the next couple of centuries depends largely in human behavior what will the rates of greenhouse gas emissions be? Many projections assume the A1B business as usual emissions scenario, which (averaging results for different climate models) is expected to yield a warming relative to the late 20 th century of little less than 3 deg. C. Source: IPCC-AR4

44 The magnitude of surface warming (response) in equilibrium with a given global radiative forcing depends on the climate feedbacks. The equilibrium response to the present-day radiative forcing is about 1.2 deg. C. The Problem is that the radiative forcing is going to grow. Forcing Feedback Response Water Vapor Feedback Ice-Albedo Feedback Equilibrium Climate Sensitivity: Around 0.75 deg. C per Watt/m 2 forcing 1.6 W/m 2 X 0.75 = 1.2 deg. C

45 Projected changes in annual averaged surface air temperature relative to the Late 20 th century for different emissions scenarios. Temperature rises are expected to be strongest in the Arctic. Arctic amplification is strongly tied to reductions in sea ice cover, allowing for strong heat transfers from the ocean to the atmosphere. This is in turn linked to the albedo feedback. Source: IPCC-AR4 See earlier slides - Arctic amplification is already here.

46 Model-Projected Arctic Amplification NCAR CCSM3 projection of 2-meter temperature anomalies by month and year over the Arctic Ocean, compared to means Latitude by height dependence of zonally averaged October March temperature anomalies for , compared to means (NCAR CCSM3) The modeled emergence of Arctic amplification. It will be most pronounced in the cold season and will have a clear vertical expression, becoming stronger From the lower tropospere towards the surface. Source: Serreze et al., 2009

47 2008 Falls Just Short Sept 14, 2008: 4.51 million sq. km Sept 16, 2007: 4.13 million sq. km The observed rate of decline in September ice extent exceeds expectations from most climate models. Based on model projections, ice-free Septembers might be expected anywhere from 2050 out to beyond Given The apparent conservative nature of the models, an ice-free ocean might be seen as early as Source: updated from Stroeve et al. (2007)

48 Near surface permafrost is expected to thaw, which may lead to release of carbon stored in the now-frozen soils, leading to further warming. Permafrost contains about 950 Gt of carbon (Zimov et al., 2006: Science). For comparison, the carbon content of Earth s atmosphere is about ~730 Gt today. Source: Lawrence and Slater, 2005.

49 Ice Loss Leads to Terrestrial Warming Atmospheric circulation is expected to spread the effects of strong heating over the ocean (Arctic amplification) to high-latitude land areas, contributing to thaw of permafrost and carbon release to the atmosphere. Source: D. Lawrence and A. Slater

50 Emerging signals of a carbon cycle feedback? Methane bubbles trapped in lake ice Lake bubbling with methane in the Arctic Burning methane over a thermokarst lake in Siberia (K. Walter) Courtesy K. Walter

51 The Arctic hydrologic cycle is Projected to intensify through the 21 st century Decadal changes in multi-model mean freshwater budget terms for the Arctic Ocean. Positive anomalies mean an increasing source (or decreasing sink) of freshwater for the Arctic Ocean. Anomalies are with respect to means for 10 models participating in the IPCC-AR4, A1B emissions scenario for the 21 st century From Holland, Finnis, Barrett and Serreze, JGR, 2007

52 Projected changes in winter (DJF) and summer (JJA) surface temperature, precipitation and sea level pressure for the period , relative to from an average of models participating in the IPCC-AR4. Results are based on the A1B emissions scenario (Source: IPCC 2007). While temperature will rise strongly in the Arctic, and precipitation will increase (but decrease on other areas), there will also be changes in patterns of atmospheric circulation. While there is come evidence that the NAM might shift to a more positive state, much is unknown. How might reduced sea ice influence the NAM? The slide that follows suggests complex changes.

53 Nov-Dec Air T Dec SLP Present-day Inversion ci = 1hPa T Oct-Mar Precipitation March Snow Depth 0 ºC Mainly in early winter Deser et al. (2009)