This blog will review what I believe is the state-of-the-science, one that will avoid hype or politicization of the issue. One based on peer-reviewed publications and the best models we have available. And an analysis that will be honest about what we don't know and the uncertainties.
What Climate Changes Have Occurred During the Past Decades in the Pacific Northwest?
Mankind has already changed the surface climate in our region in profound ways. We have massively irrigated parts of eastern Washington, reducing summer temperatures there (2-5F). A high resolution satellite image illustrates the vast irrigated areas in the Columbia Basin.
We have cut down the trees in western Washington, replacing them with concrete and buildings, substantially warming the surface (as much as 5-10F, see satellite based temperatures below). Thus, there are urban heat islands in our built up area, with enhanced local temperatures.
We have created large farms in eastern WA, tearing off the protective surface vegetation, resulting in dust storms. And our mismanagement of eastern Washington forests have resulted in increasing large fires and major smoke outbreaks.
There is more, but the message is clear: human activities have profoundly changed our surface climate in limited areas, without even talking about greenhouse gases in the atmosphere.
What do surface weather observations show, regarding changes in temperature and precipitation over our region during the past decades? Is there a trend?
Here are plots of NOAA/NWS average temperatures over Washington State for January through June for the past 50 years from their climate division data set (I chose a half year so 2017 could be available). For temperature, there was a big spike in 2015 and a secondary one in 1994, but the overall trend is small, with perhaps a degree of warming for the entire period and little trend since the late 1970s . Most of the increase occurred abruptly in the late 1970s, associated with shift in the Pacific Decadal Oscillation (PDO), which is a mode of natural variability. And there was some impact of urbanization.
Precipitation over the State shows little trend, with lots of transient up and downs.
Regional snow pack on April 1 (right before the big spring melting season) has shown little trend during the past 40 years.
Our Future Climate
But what about the future? There has been an increasingly rapid upward trend in greenhouse gases (CO2, methane, water vapor), with human-caused increases in CO2 being the dominant forcing agent. The science is clear: increasing greenhouse gases will warm the planet--but there are a lot of nuances and details that are critical.
Mankind has done little to stop the increase in CO2, with a fast upward trend in renewable energy being overwhelmed by surging use of coal, oil, and natural gas (see plot of CO2). Carbon-free nuclear energy is being rejected. CO2 concentrations are actually rising more quickly today than decades earlier.
The Arctic warms up more than anyplace else (due to melting polar snow/ice and some subtle radiative effects). The continents warm up more than the oceans (the oceans have huge thermal mass that take time to warm), and the eastern oceans typically warm up more slowly than western oceans. The Northwest, downstream of an eastern Ocean, thus will warm up more slowly than most.
Although the global climate models are our most powerful tools for looking forward, they are major issues. First, we have to assume how much CO2 and other gases will be in the atmosphere later in the century. Second, they lack the resolution to get our local climate correct. As shown below (the terrain from the NCAR climate model, CCSM4), the current generation of global models don't have the Cascades, Olympics and other critical geographical features that dominate our local climate.
Third, the climate models have major disagreements in regional areas. In a classic paper, Deser et al., 2012 (Nature Climate Change) ran an ensemble of 40 climate simulations, starting each slightly differently to simulate trends through 2060. The results for the temperatures at Seattle (see below) varied greatly, with a few runs showing no warming and others much more.
Fourth, many of the global climate models have serious flaws. Some can not realistically duplicate the current climate (e.g., double Intertropical Convergence Zones, serious biases, severe problems with organized tropical convection). And some climate scientists believe that the "tuning" to match the 20th century climate makes them overly sensitive to increasing CO2. There is a lot of dirty laundry in the climate modeling business that the general media and others are not aware of. Our current generation of forecast models can not predict seasonal changes with any skill...this is a major warning.
So what do we do? The roughly 2-dozen global models are the best tools we have. Society desperately needs guidance regarding the local impacts of climate change driven by increasing greenhouse gases.
So why not start with the best ones (the ones that match contemporary climate the best) and see what changes they have in common. And we can use the best models to figure out the local impacts by running high-resolution regional climate models that are embedded and nested in the global models. And when we do this, we have to be totally honest about uncertainties and limitations.
Let me show an example of such a regional climate model simulation for this century for the Pacific Northwest. And be prepared to be shocked.
Let's start with surface temperature (2-m temperature) for winter (DJF) assuming mankind keeps on its current track of greenhouse gas increases (a pretty good bet at this point) The change between the 1990s and 2020s is small, with all warming, but not many locations doing so by more than 3F.
A key issue is that warming is not uniform in time. The warming is slow at first (aided by the slow to warm Pacific), but by the end of the century it really revs up. Another way to see this is a plot of the number of days per decade above 90F at Sea Tac Airport.
Not much change in the 2020s, up a bit in 2050, and BIG increases during the late 2090s. For all of you under 40, you should make a note to yourself to buy an air conditioner in 2050. Maybe two.
The current NW climate has been relatively unchanged so far. But saying this has gotten me in total trouble with some local climate advocates, who call me all kinds of names for talking about this "inconvenient truth." I mean really nasty stuff, like being a "denier", a "contrarian", "dangerous", "losing my mind", and lately being sympathetic to "white supremacists". You deserve the truth, not manipulation, and exaggeration to ensure "you do the right thing".
What about precipitation in our region? Most simulations, either based on global climate models or their regional "downscaled" versions indicate that there will be a small increase (5-10%) in regional precipitation (changes over Puget Sound are shown below).
We will retain our water as the earth warms. This makes sense, a warmer atmosphere can "hold" more water, so our precipitation will be juiced up.
But occasionally, it may be juiced up more than we like. The latest simulations suggest that atmospheric rivers---plumes of water vapor coming out of the tropics/subtropics--will get substantially enhanced by global warming, and our extreme precipitation events during such rivers could be 30-40% stronger by the end of the century.
The number of heavy precipitation events will increase dramatically, particularly during the fall (see graphic below showing changes in the number of event between the ends of the 20th and 21st century).
As our region warms, the snow level will rise in the mountains, with much more precipitation falling as rain in the middle elevations, where previously there had been snow. We will have less snow in our mountains during the winter and there will be less snowpack on April 1, leaving less snow melt to provide water during the summer and autumn before the rains return.
The predicted change of snow pack by the end of the century is substantial, with losses (red colors) exceeding 40% in some locations. (see graphic)
So what do we do with water in late summer, as snowmelt lessens? Since our annual precipitation will increase, the water will be there, so we may need additional reservoir capacity in some locations. Fortunately, the critical Columbia River drainage may be in relatively good shape because it drains off the very high Rockies, where there will still be a lot of snow, and because of the huge storage capacity behind all the hydroelectric dams.
Thus lack of snow will have another negative impact: snow absorbs water during heavy rain events, so less snow means less buffering of the rain, contributing to flooding, particularly during the fall. Seriously, I would not want to live in the flood plains of our local major rivers.
What about windstorms caused by big midlatitude cyclones? The media keeps on talking about how they will get worse around here. Fortunately, the best climate simulations do NOT suggest this...our storms will be relatively unchanged (see graphic below that was produced for Seattle City Light).
Number of times per year above the 90 percentile wind speed for 1970-2000 (DJF)
The lack of change of windstorms makes sense. Our wind storms are driven by north-south temperatures differences and those will weaken at low levels under global warming (polar regions preferentially warm compared to the tropics in the lower atmosphere).
And our computer models indicate all kinds of unexpected changes in our region with global warming. For example, the amount of low clouds in the late spring and early summer may increase, as the greater heating inland causes preferential pressure falls that draw in cool, cloudy marine air (see graphic). Super June gloom powered by global warming. That will send come Californians back to where they came from!
And, dare I say something that will get me in terrible trouble with certain folks? As described in the peer-reviewed literature, warming will have some positive impacts as well, including a more pleasant climate for much of the year, less black ice on the roads, less exposure deaths of homeless folks, improved air quality during the winter, among others.
Enough. I wanted all of you to be exposed to a description of the current science not provided by some local media outlets, a few of which are providing a highly skewed, inaccurate view of what is expected. Some of outlets (e.g., Seatttlle's The Stranger) have descending into exaggeration, advocacy, and name calling, as have a few "advocate scientists.".
Society can only make good decisions only if it is given the best estimates our science can provide, and hype/exaggeration/and worse is destructive on many levels. Imagine planning a new reservoir based on exaggerated predictions! Exaggeration and hype also lead to politicization of dealing with climate change, which ensure progress is restrained.
Finally, let me note that understanding the local implications of climate change is an act in progress.
As I will describe in a future blog, several of us are trying to get better answers by running an ensemble of many high-resolution local climate simulations to get at the uncertainties and possibilities. Other atmospheric scientists (some in my department at the UW) are working on improving the global models. Others, such at the UW Climate Impacts Groups, are trying to translate the model predictions into societal implications.
There is much work to be done: in understanding the local implications of climate change, taking steps to prepare our region for what will occur, and to do what we can to lessen emissions of carbon around the world.
from Cliff Mass Weather and Climate Blog http://ift.tt/2vu1Ttx
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