Effects of climate change on water resources

Transcription

1 Effects of climate change on water resources

2 Key Points Global climate has varied widely in the past. On time scales of tens to hundreds of millions of years, these changes were at least partly a result of the shifting configurations of land and ocean and mountain-building events associated with continental drift. While it appears that the earth has experienced ice ages in many periods of its history, including the Proterozoic, 800 and 600 million years (Ma) before present, the Ordovician and Silurian (460 and 430 Ma) and the Carboniferous and Permian (350 and 250 Ma), we know little about the details of these events. Much more is known about climate changes during the Quaternary period, comprised of the Pleistocene and Holocene epochs, and extending from about 1.8 million years ago to the present. The hydrologic cycle both responded to and contributed to these changes. The observed increase in global average surface air temperature over the period of instrumental records is widely viewed as being primarily driven by increasing concentrations of carbon dioxide and other greenhouse gases. There is evidence that the hydrologic cycle is both responding to and contributing to evolving climate change, with implications for the management of water resources.

3 Geologic time scale The geologic time scale [from the Geological Society of America, product code CTS004, compiled by A.R. Palmer and J. Geissman, by permission of Geological Society of America].

4 Paleoclimate records for the Quaternary (1.8 Ma to present) Ice cores from Antarctica and Greenland (GISP2, GRIP, EPICA). Greenland records go back to the Eemian (the last interglacial, peaking about 125 years ago), Antarctic records go back 450, ,000 years. Marine sediment cores from all over the world, covering the entire Quaternary and even longer Pollen, diatoms, plant and animal macrofossils preserved in lake seiments and peat bogs Loess deposits, tree rings (dendrochronology), speleothems (mineral formations in limestone caves) Geomorphic features, such as raised beaches, moraines and glacial erratics.

5 The Vostok ice core record The Vostok record shows at least 4 major global scale ice advances over the past 400,000 years. The inferred temperature time series from oxygen isotope records is highly correlated with the ice core record of atmospheric carbon dioxide concentration.

6 Milankovitch forcings: Pacemaker of the ice ages Variations in eccentricity, axial tilt and precession (the timing of the equinoxes) affect the solar flux striking the surface at different latitudes and at different times of the year. These forcings have paced the timing of the major ice ages and interglacials of the Quaternary.

7 CO 2 and Quaternary climate change Rises and falls in temperature over the Quaternary precede greenhouse gas changes. This tells us that with regard climate changes over the Quaternary, greenhouse gases operated as a feedback, globalizing the effects of Milankovitch forcings. Today s carbon dioxide concentration (about 390 ppm) is higher than anything seen in ice core records. Today, CO 2 is acting as a climate forcing.

8 The last glacial maximum (LGM) Extent of Northern Hemisphere glacial ice during the Last Glacial Maximum (LGM) [from Denton and Hughes (eds.), 1981, by permission of John Wiley and Sons]. There were ice sheets over both North America and northwestern Eurasia. Global sea level was around 120 m lower than today. The ice sheets themselves likely affected patterns of weather and precipitation

9 Rapid climate change events Ice core records for the Quaternary document periods of very rapid climate change, the last of these being the Younger Dryas, a rapid transition back to cold conditions when coming out of the LGM. The figure above shows calcium concentrations (ppb) covering the period ka based on GISP2 ice core data. The sample resolution is approximately 2 years through the Holocene, a mean of 3.48 years within the YD (Younger Dryas) and BA (Bolling/Allerod), and 3-15 years during the OD (Older Dryas) [from Mayewskii et al., 1993, by permission of AAAS]. The rapid change to high calcium and dust concentrations during the YD point to an intensified atmospheric circulation over continental regions and increased aridity.

10 The Younger Dryas as a hydrologic event The Laurentide Ice Sheet and the routing of overflow from the Lake Aggasiz basin (dashed line) to the Gulf of Mexico just before the Younger Dryas (a) and routing of overflow from Lake Aggasiz through the Great Lakes to the St. Lawrence and northern North Atlantic during the Younger Dryas (b) [from Broecker et al., 1989, by permission of Nature]. Massive discharge of freshwater into the North Atlantic from the melting Laurentide Ice Sheet could have disrupted the thermohaline circulation, initiating the YD event.

11 Changes over the past 1300 years Records of Northern Hemisphere temperature variations over the last 1300 years. Panels are (top) annual temperature over the instrumental record, (middle) reconstructions using various proxies, (bottom) overlap of all proxy records in middle panel with shading indicating level of agreement between the different reconstructions. The observed temperature record in the bottom panel is shown in black [Source; IPCC-AR-4, Working Group I Report, Figure 6.10]. The Medieval Warm Period from about seems to have had it strongest expressions over the Northern North Atlantic sector. The Little Ice Age is dated anywhere from to The GSIP-II Greenland ice core records put the maximum cooling from

12 Global temperature change and CO 2 NOAA The global average surface air temperature has risen over the period of instrumental records, and most scientists believe that the main culprit is the radiative forcing from rising concentrations of CO 2 and other greenhouse gases

13 Radiative forcing 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 Source: IPCC to warming. A negative forcing leads to cooling. Human activities have led to an estimated positive radiative forcing of 1.6 W m -2. Source: IPCC-AR4

14 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

15 Equilibrium climate sensitivity 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

16 Hindcasted and projected global temperature change The projected global mean annual average temperature change over the next couple of centuries depends largely in human behavior what will the rate of greenhouse gas emission 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

17 Projections from the IPCC Projected changes in winter (DJF) and summer (JJA) surface air 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 averaging together results from the suite of models participating in the IPCC (Source: IPCC 2007). Temperatures are expected to rise most strongly in the Arctic. The high latitudes are expected to see a general increase in precipitation but decrease is expected to decease in others. There will also be changes in patterns of atmospheric circulation; a number of models project that the Arctic Oscillation will have a greater tendency to be in its positive mode.

18 Projections from the IPCC IPCC AR-4 Regional projections are fraught with with uncertainty. The figure at left shows projected changes near the end of the 21 st century for temperature and precipitation over North America, averaging together results from models participating in the IPCC AR-4 with the A1B scenario. The panels at the bottom indicate the number of models that agree with respect to the sign of the model-mean change. While all models indicate that it will get warmer, there is much less agreement as to the sign and magnitude of precipitation changes over North America. With respect to managing western water resources, the combination of warmer and drier would be a big problem.

19 Climate change and runoff (Box 3-4) Water balance: R = P ET Runoff ratio: w = R/P, hence R = w.p, now substitute, to get ET = P.(1-w) Consider changes in P and ET P 1 = p.p o, ET 1 = e.et o, where P 1 and ET 1 are new values, P o and ET o are initial values, and p and e are proportional changes We now substitute to get a proportional change in runoff, r r = R 1 /R o = (P 1 -ET 1 )/(P o -ET o ) = (p.p o e.(1-w).p o )/(P o (1-w).P o ) r = (p-e.(1-w))/w

20 Climate change and runoff (Box 3-4) From the previous slide, r = (p-e(1-w))/w Assume that ET does not change (increment e = 1); the case could be that increased CO 2 leads to more and bigger plants (more ET) but also a decrease in stomatal size (less ET); the two effects cancel. r = (p-e.(1-w))/w, but e is 1, so r = (p 1 + w)/w Assume that precipitation increases 20%, so incremental p = 1.2 Assume a runoff ratio of 0.2 for desert and 0.8 for the Pacific NW r desert = ( )/0.2 = 0.4/0.2 = 200% increase r pnw = ( )/0.8 = 1.0/0.8 = 125% increase Obvious shortcoming: while both P and ET may change, so may the runoff ratio! Not let s consider the Colorado River.

21 The Colorado River The Colorado River Compact of 1922 assumed the following water allocations: Upper Basin: 7.5 MAF (Million Acre Feet) Lower Basin: 7.5 MAF Mexico: 1.5 MAF A big issue: Allocations were based on flows at Lee s Ferry, Arizona. The average flow of 16.2 MAF corresponds to one of the wettest periods of the past 400 years! We know this from tree ring reconstructions; tree ring width and precipitation are correlated. As a result, the Colorado River is overappropriated. The present-day average flow is more like MAF. Climate change projections for the 21 st century: Colorado Basin will see increased temperatures and decreased precipitation, leading to decreased runoff

22 What might happen to the river flow? The Colorado River Take the equation we have already used: r = (p-e.(1-w))/w Consider a small increment change in precipitation (0.97, 3% reduction) and evaporation (0.98, a 2% reduction) but also a reduction in the present-day runoff ratio (w = 0.113, down 1.3%). Plug in the numbers: r = ( (0.887))/0.113 = 0.1/0.113 = 0.88 or 88%, meaning that runoff drops by 12%, or about 1.8 MAF using a 15 MAF baseline. We have a problem Will Lake Mead go dry? S = input output, S = change in lake storage, input = Colorado River, output = allocations and ET Barnett and Pierce (2008): estimate a 50% change in storage by the year 2021, but this is very controversial..

23 Future changes in timing of snowmelt Stewart et al., 2004 Changes in the timing of snowmelt over the western U.S. and Alaska using a hydrologic model and assuming the middle-of-the-road A1B emissions scenario. CT means center timing the timing of when half of the seasonal snowpack mass is gone. Seasonal snowmelt is projected to occur earlier in response to higher temperatures, meaning earlier spring peaks in streamflow.

24 Observed changes in 1 April snow water equivalent Observations indicate a general decline in 1 April snow water equivalent (SWE) at measuring sites across the western U.S.; the VIC hydrologic model simulates the basic features of the observed change. The SWE on 1 April tends to be the approximate maximum seasonal value and is a key to water management strategies. Mote et al., 2005

25 Observed changes in streamflow and snow There is some tendency for the spring melt pulse onset to occur earlier in the year. Stewart et al., 2005

26 Observed changes in snowmelt timing in Colorado Day of Year Year The day of year at which half of the mountain snowpack has melted, based on Colorado SNOTEL stations with long records. The date is slowly getting earlier with time (the snowpack is melting our earlier) although the trend is certainly rather tenuous at present. From Clow, 2010

27 Large Interannual Variability While the downward trend in April 1 snowpack water equivalent is troubling, a dominant concern in water management strategy at present is the large year to year variability in mountain snowpack water storage. This is illustrated in the panels above which show snow water equivalent anomalies for April 1 of 2002 and 2005.

28 Tropical storms and climate change There is evidence, albeit controversial given issues of data quality in the early part of the records, that there has been an increase in the percent of tropical storms of the stronger categories (categories 4 and 5), that is linked to warming of the oceans, and that this trend will continue and the oceans continue to warm.