National Climate Assessment: The Future

The National Climate Assessment released yesterday (see the previous post) summarizes some of the key trends that are expected to emerge or accelerate during the 21st century.

It's no surprise that warming is expected to continue in the coming decades, with an increase in the frequency and intensity of heat waves and a decrease in the frequency and intensity of cold waves.  This is consistent with recent climate trends, our understanding of the climate system, studies of past climate, and climate-model projections.

The magnitude of the warming is dependent on future greenhouse gas emissions.  The figure below shows the change in average annual temperature from 1970–1999 to 2071–2099 projected by climate models based on scenarios with rapid greenhouse gas emission reductions (left) and continued emissions increases (right).

Source: http://nca2014.globalchange.gov/report/our-changing-climate/future-climate-change

These reports are sometimes called by some as "alarmist," but the summary statement in Chapter 2: Our Changing Climate is actually subdued.

"Global climate is projected to continue to change over this century and beyond. The magnitude of climate change beyond the next few decades depends primarily on the amount of heat-trapping gases emitted globally, and how sensitive the Earth’s climate is to those emissions."

For a summary statement, it strikes me that they should have been more specific about the magnitude of the warming (with assessment of likelihood) and the fact that warming and related changes to the climate system will likely continue well beyond 2100 unless we reduce greenhouse gas emissions dramatically and rapidly.  For example, here's what the IPCC said in their 2013 Fifth Assessment Report.  RCP2.6 is the rapid greenhouse gas reduction scenario, with 4.5, 6.0, and 8.5 representing low, moderate, and high emission scenarios.

"Global surface temperature change for the end of the 21st century is likely to exceed
1.5°C relative to 1850 to 1900 for all RCP scenarios except RCP2.6. It is likely to exceed 2°C for RCP6.0 and RCP8.5, and more likely than not to exceed 2°C for RCP4.5. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Warming will continue to exhibit interannual-to-decadal variability and will not be regionally uniform."

Water is the agent that delivers climate-change impacts, to projections for changes in precipitation, including the average climate and the intensity and frequency of storms, are also quite important.  As summarized in the assessment, precipitation projections for the nation, based on an average of many climate models, call for drier future conditions across most of the southern continental US and wetter future conditions across most of the northern continental US.  Hatched areas indicate that the trends are significant and consistent amongst the models.

http://nca2014.globalchange.gov/report/our-changing-climate/future-climate-change

Those with an astute eye can probably infer that Utah lies in the transition zone between the wetter and drier climate regimes in an area that is not hatched, indicating a lack of consistency amongst the models.  Although it is often said that the Southwest is expected to dry with climate change, trends for Utah, especially northern Utah, are a bit less clear.  Due to a "reved up" water cycle, heavy precipitation events are expected to increase in frequency and intensity.

The big wildcard in this climate assessment game is sea level rise.  This can easily be ignored when you live in a high-altitude state like Utah.  Although some say that there will be winners or losers with climate change, but everyone loses with sea level rise.  Sea level rise stems from the warming of the oceans, which results in thermal expansion, and the loss of land-based ice.  As discussed in the sea level rise section of the assessment, thermal expansion alone is projected to produce a rise of about 11 inches in low emissions scenarios, with high emissions scenarios yielding larger rises.

The challenge, however, is estimating the rate of ice loss from the Greenland and Antarctic ice sheets, which house the vast majority of the land-based ice.  Thanks to the GRACE satellite, we now have remarkable estimates of ice mass loss from Greenland and Antartica.  At present, both are showing declines that if sustained would add about 18 inches to sea level rise by 2100 (this is in addition to increases from thermal expansion and other land-based ice).

http://nca2014.globalchange.gov/system/files_force/downloads/low/NCA3_Full_Report_Appendix_4_FAQs_LowRes.pdf

The challenge, however, is that the record is short and efforts are needed to determine if these trends are due to long-term changes or short-term variability and how they will change in the future.  Ultimately, the authors of the National Climate Assessment conclude that a foot of sea level rise is on the low end of possibilities, with four feet plausible on the high end, although they note that decision makers might want to use a wider range of 8 inches to 6.6 feet.

http://nca2014.globalchange.gov/report/our-changing-climate/sea-level-rise

Of course, sea level rise will not stop in 2100.  Although not highlighted in the report, one of the more important passages in my view is the following.

"Sea level rise will not stop in 2100 because the oceans take a very long time to respond to warmer conditions at the Earth’s surface. Ocean waters will therefore continue to warm and sea level will continue to rise for many centuries at rates equal to or higher than that of the current century. In fact, recent research has suggested that even present day carbon dioxide levels are sufficient to cause Greenland to melt completely over the next several thousand years.

Once the train leaves the station, it will be impossible to stop.

National Climate Assessment Released

The U.S. Global Change Research Program (USGCRP) coordinates and integrates federal research on climate change and its impact for the nation.  As mandated by the Global Change Research Act of 1990, the USGCRP also produces a report to the President and Congress known as the National Climate Assessment (NCA) every four years.

Today, the USGCRP released its Third National Climate Assessment, which is available for download or web browsing (see also coverage by the Capital Weather Gang).  Given the release of the IPCC Fifth Assessment Report just a few months ago, the NCA doesn't contain anything all that surprising, but does provide yet another perspective on our warming world.

One of the key areas of improved scientific understanding in the past several years concerns the role of human and natural influences in driving global climate change.  As summarized in Chapter 2: Our Current Climate:

"Natural drivers of climate cannot explain the recent observed warming.  Over the last five decades, natural factors (solar forcing and volcanoes) alone would actually have led to slight cooling.  The majority of the warming at the global scale over the past 50 years can only be explained by the effects of human influences, especially the emissions from burning fossil fuels (coal, oil, and natural gas) and from deforestation."

Multiple lines of evidence support this conclusion, including climate model simulations with and without human factors shown below.

Observed global average changes (black line), model simulations using only changes in natural factors (solar and volcanic) in green, and model simulations with the addition of human-induced emissions (blue). Climate changes since 1950 cannot be explained by natural factors or variability, and can only be explained by human factors. (Figure source: adapted from Huber and Knutti).  Figure and caption source: Walsh et al. (2014), The Third National Climate Assessment.

Additional indicators of climate change in the United States discussed in the report include:

• An increase in the average temperature of 1.3–1.9ºF since 1895 (most of this increase has occurred since 1970), with the last decade the nation's warmest on record.
• An increase in the frost-free growing seasons since the 1980s, with the largest increase over the western United States.
• An increase in heavy downpour frequency, especially in the last three to five decades with the largest increases in the midwest and northeast.
• More frequent and intense heat waves and less frequent and intense cold waves.  

The report also notes that the intensity and duration of North Atlantic hurricanes, and the frequency of Category 4 or 5 hurricanes have increased since the early 1980s, but that the relative contribution of human and natural causes to these trends is uncertain.  Trends in other severe storms (tornadoes, hail, damaging thunderstorm winds) are also uncertain.

One conclusion that I think may be misleading is "winter storms have increased in frequency and intensity since the 1950s, and their tracks have shifted northward over the United States."  This statement is based on trends in extratropical cyclones, low-pressure systems that develop in the mid and high latitudes, and not trends in low temperature precipitation events that produce snow, sleet, or freezing rain, which is what I think of when I hear the term winter storm.  Although I suspect the authors were trying to avoid the use of a jargony term like extratropical cyclone, this was an instance where they probably should have been more specific.  To date, trends in the frequency of heavy snowstorms and seasonal snowfall show considerable regional variability and are difficult to generalize for the nation (this is discussed in the relevant sections of the report).  

Perhaps we'll have a look at the projections for future climate change in a forthcoming post.  

Dust on the Snowscape

It's looking like this spring will yield a dusty and dirty snowpack in the Wasatch Mountains, thanks to the April 22nd dust storm from the Sevier Lake Bed (see Sevier Lake Bed Dust) and several minor events over the past few weeks, including yesterday.

Most of the dust was buried temporarily by recent storms, but given the heat of the past few days, it's now being exposed, especially in sunny areas where the new snow was thin due to wind scouring or sloughing.  Here are a few perspectives of the snirt (part snow part dirt) from Saturday.

Snirty patches at Alta Ski Area 

Snirty slough zones on Little Superior

A snirty Mount Superior

Dusty snow absorbs more sunlight than clean snow, accelerating the snowmelt.  The comparison below from the San Juan Mountains of Colorado shows that aged (i.e., not fresh), clean snow reflects about 72% of the sunlight back to space, whereas dusty snow reflects only about 43%.

Courtesy Tom Painter, JPL

At noon in mid May in the San Juan Mountains, the clean snow absorbs about 308 Watts of solar energy per square meter, whereas the dusty snow absorbs 627, 319 Watts per square meter more.  That's a huge increase.  Dust in the Wasatch isn't as dark or typically concentrates as found in the San Juan's, so the difference here might not be quite as large, but it is still going to be substantive.  The bottom line is that dust accelerates the snowmelt and reduces the duration of snow cover in the spring.  I would imagine that increased absorption of solar radiation likely results in an earlier, more rapid increase in avalanche hazard on sunny mornings as well.

Living in Utah Has Its Privileges

May 3, 2014

I often tell people that a bad year in Utah is a better than a good year in Colorado.  This year would likely qualify as a bad year by Utah standards, with Alta reporting a seasonal snowfall of only 432 inches, well below the long-term average of 510 inches.  Nevertheless, Utah is where a bad year still means a 105" base on May 3rd. 

The skiing today was typical of a hot day in the spring.  Everything from terrifying hard pack to sublime corn to to manky slush depending on the aspect and the time of day.  High Rustler was still somewhat firm when we tempted fate on it a bit after 11.  Look carefully at the photo below and you can see the dust emerging on the south facing aspects on the far side of the canyon.  More on this in the near future.

Last Rustler run of the season.  Bon voyage mon ami.

In the early afternoon, it was quite hot.  Looks like a high of 56ºF at 9700 feet and 62ºF at the Alta base.  Clothing optional for some. 

Tomorrow looks warm with a bit more wind for Alta's closing day.  I suspect there will be more displays of plumage.

Is It Getting Dustier in Northern Utah?

People frequently ask me if there has been a recent increase in dust storms over northern Utah.  This is actually a difficult question to answer because nobody has been directly measuring atmospheric dust concentrations for more than a few years.  As a result, we need to piece together what we can using what evidence is available.

For long-term trends, our best evidence comes from cores taken from alpine lakes.  These cores allow us to infer accumulation rate and composition of sediments going back for many centuries. Cores collected in alpine lakes in the San Juan Mountains of Colorado show a dramatic increase in dust accumulations beginning in the late 1800s. 

Sediment and mass accumulation rates from Porphyry Lake,
San Juan Mountains, CO.  Source: Neff et al. (2008).

Cores from alpine lakes in the Uinta Mountains show similar trends (see Reynolds et al. 2009).  This evidence, combined with analysis of the sediment composition, supports the conclusion that there was a significant increase in dust arising from land disturbance following the western settlement of the United States in the late 1800s.  Such land disturbance includes agricultural and livestock grazing, mining, and other activities.  There is some evidence of a decline in dust accumulation after the 1930s, possibly related to the enactment of the Taylor Grazing Act of 1934, although accumulation rates remain far higher than occurred before western settlement.  

These lake cores, however, don't allow us to examine the frequency and characteristics of individual dust storms and how they change from year to year and decade to decade.  To get at such fine-scale details, one option is to use hourly surface observations from the Salt Lake City International Airport, which provides a nearly continuous record back to 1930.  That sounds great, but there are challenges with using this record, including inconsistencies in reporting practices and observer biases, coding and translation errors, and changes in instrumentation, reporting guidelines, and processing algorithms.   Keep these limitations in mind as we plow forward.

The number of dust events reported at the Salt Lake City Airport does show an over decline since 1930, with large ups and downs from year to year.  A relatively small number of events were reported around water year 2000 (the water year is October to September), with a greater frequency of events since that time.  

Source: Steenburgh et al. (2012)

At issue with such an analysis, however, is that it doesn't account for event intensity.  One big event have a huge impact on dust accumulations.  Without direct dust observations, we are forced to improvise and use visibility as a proxy for dust concentration.  If we do this, and consider the strength of the winds, we can estimate the total flux of dust each water year at the Salt Lake City Airport.  This shows even larger year-to-year variations since intense events are less common but have a huge impact on the total dust flux.  Nevertheless, there is some downward trend with the minimum apparent again around water year 2000.

Source: Steenburgh et al. (2012)

Now is the time to inject some word of caution.  Recall some of the issues at play in this dataset.  When combined, however, with the alpine lake core data, it is probably safe to say that overall dust accumulations are somewhat lower today then they were in the 1930s.  If we could freeze the current climate and land-use conditions, we'd probably see ups and downs from year to year and decade to decade similar to those since the 1970s.  The future, however, will depend largely on land-use management, water-resource management, and climate variability and change.  

It is worth noting that dust emissions are not equal across the Great Basin.  The image below shows the approximate origins (plus signs) and orientations (colored lines) of major dust plumes identified in satellite imagery western Utah, Nevada, and southern Idaho.  

Source: Steenburgh et al. (2012)

It is possible that the lowest hanging fruit for reducing the frequency and intensity of wind-borne dust events would be to target these areas of dust emissions.  

Oh Sill Hill

The Santa Ana wind event in California has brought some impressive winds to the mountains of southern California.

Sill Hill is located just to the west of Cuyamaca Peak and observed some intense downslope winds on the 29th and 30th, with a peak gust of 101 mph.

Note the strong day to night variations in wind speed, with winds increasing overnight, reaching a maximum in the morning, and then decreasing during the afternoon.  This is very similar to the behavior of downslope wind storms along the Wasatch Front.  

The map below shows the situation just before Sill Hill hit 101 (Sill Hill is the site with an east wind and a gust, indicated by the numbers, of 98 mph just left of center in the image).  Note the intense flow on the west side of Cuyamaca Peak.  

I'm off to graduation festivities.  Congrats to all the new U of U graduates.