The Drying Climate in the American West 63 rd Annual New Mexico Water - - PowerPoint PPT Presentation

Brad Udall Senior Scientist/Scholar Colorado State University Bradley.Udall@colostate.edu

The Drying Climate in the American West

63rd Annual New Mexico Water Conference October 17, 2018 Las Cruces, NM

Lake Creek, Colorado September 8, 2018

https://bit.ly/2NKGnsD

Water Year 2018 Temperature Water Year 2018 Precipitation

Key Points

• Climate Change already impacting the basin

– Temps for sure, maybe precipitation

• Impacts will get Worse

– “New Normal” inadequate to convey challenges – Aridification underway – not a drought

• Plan on…

– Heat! More and More as the Century Proceeds – Shifting runoff patterns

• South (Dry) and North (Less Dry to Perhaps Wet)
• Earlier within-year runoff

– More WX Variability

• year to year, within-year

– Substantial Flow Reduction Risk – Substantial Megadrought Risk – Flood Risks

• Localized – Likely
• Basin-wide - ??

– Higher Water Temperatures – Fires

• Opportunity for Change

The 2000-18 Millennium Drought is very, very different from previous droughts in the historic record…

Represents 6 maf loss

• ver 18 years relative to

worst 20th century drought

Millennium Drought 2000—2018

• 2000-2017 is the worst

drought in the gaged record

• ~ 20%/yr decline
• Long-term trend, too

Udall and Overpeck, WRR, 2017

Millennium Drought 2000—2018

• 2000-2017 is the worst

drought in the gaged record

• ~ 20%/yr decline
• Long-term trend, too
• Lakes Powell and Mead

have lost 50% of their volume

Udall and Overpeck, WRR, 2017

Millennium Drought 2000—2018

• Precipitation declines
• nly partially explain
• ~ 2/3 of the loss

Udall and Overpeck, WRR, 2017

Millennium Drought 2000—2018

• Precipitation declines
• nly partially explain
• ~ 2/3 of the loss
• Temperature increases

explain the remainder

• ~ 1/3 of the loss
• Why?
• More Evaporation
• Thirstier

Atmosphere

• Temperature-Induced

Losses

• Now = ~6%
• 2050 = ~20%
• 2100 = ~35%

Udall and Overpeck, WRR, 2017

Udall and Overpeck, WRR, 2017

• Temperature can be a major flow driver
• Since 1988 flows have been less than expected given winter precipitation
• Warm temperatures exacerbated modest precipitation deficits in the

Millennium Drought

Model-based Study using Historical Data

– Long-term Trend Analysis (-16.5% Decline) – Temperature De-trend Model Experiment – 1950s vs 2000s Drought Analysis – 2017 Forecast Analysis

• Findings

– ~50% of Decline due to Higher Temperatures – ~50% of Decline due to Changing Precipitation Patterns

4 Key Basins (Green + Blue) produce ~55% of all runoff

Denver Water Forecast Inflow vs Runoff 2018

• Colorado River snowpack peaked at 107% of normal
• South Platte River snowpack peaked at 85% of normal

South Platte at South Platte Blue River at Dillon Williams Fork Fraser at Winter Park May 1 NRCS Forecast 48% 99% 81% 99% Actual Natural Volume 38% 71% 71% 77%

April through July Natural Streamflow Percent of Average

Source: Denver Water

• 22

%

Denver Water Forecast Inflow vs Runoff 2018

• Colorado River snowpack peaked at 107% of normal
• South Platte River snowpack peaked at 85% of normal

South Platte at South Platte Blue River at Dillon Williams Fork Fraser at Winter Park May 1 NRCS Forecast 48% 99% 81% 99% Actual Natural Volume 38% 71% 71% 77%

April through July Natural Streamflow Percent of Average

Source: Denver Water

• 22%
• 10%
• 28%
• 10%

New Normal?

Dave Pierce, Scripps

It clearly will get hotter Expansion of the Wet/Dry Axis = more variability Hint of more occurrences

• f extreme wet than

extreme dry Note: precipitation is not runoff…

Aridification – not a drought

• Declining Snowpack and

earlier runoff

• Higher Temperatures
• Drying Soil
• Thirsty Atmosphere
• Moving storm tracks
• Shorter Winter/Longer Fall
• Greening ?
• Megadrought?

Seager et al., 2018

Aridity Index (P/PET) Changes

Climate models: north is wetter, and south is drier Wet: Wind Rivers + Unita Mountains Dry: Most of Colorado Key: Where is the ‘hinge’ point? CRB Mainstem critical

Changing Precipitation Patterns: Dry South, Less Dry North

Changes in Soil Moisture in 2100

The Upper Colorado River Basin is Megadrought Country – 1200 years of

Colorado River flow thanks to tree rings

Meko et al., (Geopysical Research Letters, 2007)

Cook Science

In both Central Plains and Southwest, Multi- decadal Drought Risk* exceeds 80% in 21st Century * Defined as Drought lasting 35 or more years Percent Chance

• f Multi-

Decadal Drought Risk, Southwest US

P = +10% P = +20%

P = +10% P = +20% Risk = 70% w/ +10% Risk = 35% w/ +20%

Other Supporting Studies

• Response of Colorado River runoff to dust radiative forcing in

snow – Painter, et al., 2010

• Hydrologic Sensitivities of Colorado River Runoff to changes in

Precipitation and Temperature – Vano et al., 2014

• Climatology, Variability, and Trends in the U.S. Vapor Pressure

Deficit, an Important Fire-Related Meteorological Quantity – Seager et al, 2015

• Running Dry: The US Southwest’s Shift to a drier climate

– Prein et al., 2016

• Relative impacts of mitigation, temperature, and precipitation
• n 21st-century megadrought risk in the American Southwest

– Ault et al., 2016

• Evidence that Recent Warming is Reducing Upper Colorado

River Flows – McCabe et al., 2017

• The Curious Case of Projected Twenty-First-Century Drying but

Greening in the American West – Mankin et al., 2017

• Dramatic declines in snowpack in the western US

– Mote et al., 2018

• The More Extreme Nature of North American Monsoon

Precipitation in the Southwestern United States as Revealed by a Historical Climatology of Simulated Severe Weather Events – Luong, et al., 2018

Vapor Pressure Deficit Trends

Seager et al., 2015 Mote et al., 2018

Snow Water Equivalent Trends

Key Points

• Climate Change already impacting the basin

– Temps for sure, maybe precipitation

• Impacts will get Worse

– “New Normal” inadequate to convey challenges – Aridification underway – not a drought

• Plan on…

– Heat! More and More as the Century Proceeds – Shifting runoff patterns

• South (Dry) and North (Less Dry to Perhaps Wet)
• Earlier within-year runoff

– More WX Variability

• year to year, within-year

– Substantial Flow Reduction Risk – Substantial Megadrought Risk – Flood Risks

• Localized – Likely
• Basin-wide - ??

– Higher Water Temperatures – Fires

• Opportunity for Change

Increasing Heat Content Everywhere

90% into the

• cean

FOLU = Forestry, Other Land Use

Climate Change is Water Change

• Heat Drives the Water Cycle –

1000 km3 evaporates daily from the oceans

• The Water Cycle mixes heat from areas of too much to too little
• As the Atmosphere Warms it Holds More Moisture:

~5F warming is 20% increase

• Heating Up the Earth (and uneven heating) results in Water Cycle changes

– More Evaporation, More Precipitation, More Moisture – Changes in weather patterns – Wet Wetter, Dry Drier Standard Rule – More Intense Floods and Droughts

• All Kinds of Water Changes Already Noted

– More rain/less snow, Earlier Runoff, Higher Water Temps, More Intense Rain

American Geophysical Union Fall Meeting – San Francisco

• December every Year
• 24,000 Earth Scientists
• Fully Accepted Science

– Earth is Warming – Humans are the Cause – Very Risky to Burn Carbon unabated – Water Cycle will be Impacted

• Theory vs “Belief”

Almost every Major Scientific Organization has statement affirming Anthropogenic-caused Warming AND the need to act…

• National Academy of Science
• All International Academies of Science
• American Association for the Advancement of

Science

• American Geophysical Union
• American Meteorological Society
• The Geological Society of America
• Many, many others..(but not AAPG)

Climate Change Myths

• Climate is always changing
• It’s the sun
• You said it was going to cool in the 1970s
• Winter Related

– It’s cold and snowy – We just set a cold record

• Can’t predict weather, so can’t predict climate
• Models are not any good…
• 1930s were warmer...
• It’s the Urban Heat Island Effect
• It’s volcanoes

Climate Change a combination of …

• 1. For-Sure Temperature

Rise -> Flow Losses

• 2. Not-Sure Precipitation

Change -> Flow Gains or Losses

Colorado River Future Flow Losses

In the future, plants move up slope, increasing evapotranspiration. By 2100 ET increases by 28% and Kings River flows decline by 26%.

Evapotranspiration Increases in Natural Systems

Now Future

Colorado River Drought 2000-2014

Sources: Udall and Overpeck, 2017; Woodhouse et al., 2016

• 2000-2014 Worst Drought in

Colorado River Gage Record

• ~ 1/3 of the Decline due to

Higher Temperatures

• 20% Loss by 2050 Possible

due to higher temperatures

• Increases in precipitation

may counteract losses somewhat

• Increased risk of

megadrought in 21st century reinforces loss potential