Dr. Dan Cayan is the director of the Climate Change Center at Scripps and also concurrently holds a research position in the USGS Water Resources division. At the November meeting of the Delta Stewardship Council, he gave an overview of California’s climate variability and of the current state of knowledge of the potential impacts of climate change on the state’s water resources.
Dr. Cayan began by presenting a slide showing precipitation by water year, going back to about the turn of the century. “The thing that is striking is the amount of ups and downs both on a year to year basis and also along the smooth curve there, there are episodes of relatively wet and relatively dry,” he said. “The last three years have been decidedly dry – actually the last 16 years or so. Since the late 1990s, we’ve been dry in California.”
He then presented a chart that shows the observed precipitation departure from 1998-99 through 2013-14. “If you tally this up, either gains or losses, the warm shadings are losses per year,” he said. “You can see the bulls eyes that occurs largely over California and Arizona. It is about a year and a half to over two and a half years of normal precipitation that we have not received over the 16 year period, so the point is that while these last three years have been quite dry, we’ve actually been subject more or less to dryness over this period, and really since the last really large El Nino that occurred in the Pacific basin in 1998.”
“We are dependent exquisitely on heavy storm events,” he said, presenting a chart of an atmospheric river storm. “The tropical Pacific is the pool of not only warm water, but also really moistened atmosphere, and occasionally we get these filaments of high water vapor that penetrate into the extra-tropics and these are the storm generators, the really large ones along the west coast. In some cases, they are flood causing events, but they are also really crucial in delivering the California water supply. This was observed in 2004 where you can see one of these atmospheric river events that extends from the tropical Pacific with associated strong winds. When those winds intercept mountain slopes, of course, we get copious amounts of precipitation, either rain or snow.”
Dr. Cayan then put up another slide and directed everyone’s attention to the graph at the bottom which shows the number of pineapple express storms making landfall. “I’d like to draw your attention here at the bottom to the bar graph that shows the number of that type of event per year, and it’s really just a handful of these events that occur every year that California is exposed to. That’s not a regular phenomenon so it varies quite a bit from one year to the next. For the last three years, we’ve been lacking in these kinds of storms. If you go back to the year 2011, we had some big ones, and so forth, and the interesting thing is that if you plot the number of these events, which is the orange curve at the top, and overplot it on the amount of water year precipitation, that’s the black trace on that same upper plot, there’s a remarkable correspondence.”
“In California, we’re very strongly dependent on these large storms in delivering our water supply, and if you map that out, the dependence of large storms, it’s not exclusive to California,” he said. “There’s well over 50% of the variability across the U.S. is owing to a big handful of storms, but in California, it’s an acute dependence. That’s represented by these darker spots on the map which show that we are above the average in our dependence on these very large storms.”
Dr. Cayan then turned to climate change. He said that when they look at climate change, they don’t look at any single particular model, but instead use a combination of over 40 models. “Just like stock market pundits, we tend not place too much trust on an individual, but we look at the consensus,” he said.
He presented a slide showing projections for temperature change in Sacramento. “What this is showing is the consensus view of summertime temperature over Sacramento from a set of these climate simulations. The dark line is the median of a number of these climate simulations. The models are run retrospectively – that’s the left hand portion of the chart, and then prospectively from present day forward. They are run using assumptions of essentially human influences, including carbon dioxide and other greenhouse gases in the atmosphere.”
Dr. Cayan explained that the red line is for a relatively high level of greenhouse gas emissions, the brown line is an intermediate level, and the green line represents a very optimistic scenario for emissions. “What you can see is that as the decades proceed, we are crossing the level of temperature change that exceeds 3 degrees Celsius,” he said. “That doesn’t sound like a whole lot, it’s about 5 degrees Farenheit of increase over historical levels, but it’s a 24/7 level of temperature change averaged over a month, in this case July. The scientific community has determined that something between 2 and 3 degrees Celsius is what we’re calling a dangerous level where as thresholds get crossed, it becomes harder and harder to adapt from a variety of perspectives.”
“We have sea level problems, we have melting snow and ice, and a cascade of effects in global climate, so this shows that even under the best of circumstances, we’re flirting with these higher levels of temperature change, and under the actually presently occurring trajectory of greenhouse gas emissions, we are far exceeding that level by the end of the century,” he said. “You can see temperatures here that approach probably ten degrees Fahrenheit of an increase in Sacramento. We hope, of course, that doesn’t happen, but it’s a scenario that we have to take seriously.”
“One of the concerns is the effect of a warmer climate on the California snow reservoir,” he said, presenting a slide showing projections for the loss of California snowpack from 21st century warming in the years 2030, 2060, and 2090. He explained that blue is representing changes that are not too different from today’s levels, but yellows and reds are changes approaching 50 to 70% of today’s levels. “You can see that in the early part of the century over a 30 year average, we’re losing only about 5% of the present day spring snowpack by this particular model simulation, but by mid century, we’ve lost about a third. The losses are occurring on the fringe of the Sierras, in particular elevations that are low to intermediate that in today’s climate that do receive a snowpack, but in the future, as climate warms, that will be vastly diminished. Finally by the end of the century, under this scenario, we’ve lost over half of the present day snowpack.”
“This is relatively kind scenario,” Dr. Cayan noted. “This is a warming that achieves about 2 degrees Celsius of temperature increase by 2100. Of course, there are several model simulations where the amount of warming is in excess of that.”
“This doesn’t mean we’re losing water,” he said. “It means that we’re losing storage as snow, so in this case, if you looked at the precipitation variability over time, it would not look too much different as today. It goes up and down and so forth, but the net change, over the longer period is not too much.”
He then presented a slide showing the diminishing odds of achieving a median snow pack. “We’ve stacked up over 30 of these climate simulations and put at them in a distribution, and this is the odds of achieving median today’s level snowpack,” he said. “In the historical period, which is the left hand element of this chart, the odds are 50/50, that’s by definition, but as time goes on, of course, those odds decline. In fact by the end of the century, the odds of achieving a median snowpack are reduced to about 10%, so instead of one of every two years, we would get median at one of every 10.”
“Then on the bottom graph, I’m looking at the odds of the chance that we might only receive 10 percentile snowpack or below, and of course those odds increase over time and by the end of the century, what used to be a 1 in 10 year event, is now a 4 in 10 year event,” he said.
“Today, you’re up to your ears with the present drought and so forth, but the signals that we’re getting from climate science and climate modeling are that there are going to be even greater challenges to contend with,” he said. “I would say that underscores and reinforces the kinds of decisions and kind of activities that you promote in the Council, so I think the work here is extremely important.”
“I neglected to say that I’m not an individual that works on this; I represent really a collective. There is a very strong science community in California as you well know, and I think importantly, the science community has been engaged, and there’s a very active effort to understand the changes that we see here,” he concluded.
Chair Randy Fiorini noted that the statute that created the Council requires them to consider a 55” rise in sea level by the year 2100. “I’m always hoping for someone to tell us that that’s maybe on the high end. You today suggested that your warming predictions are kind,” Mr. Fiorini said.
“55” is at the high end of projected sea levels,” Dr. Cayan replied. “I would say that this has been a bit of a moving target for decision makers because the science is developing for sea level projections. … the middle range in the science community for global sea level rise is about 3 feet, so 55” is at the higher end of the projections, but it’s not outside of the range. … The state has taken prudent posture on this in terms of warning about the outer possibilities.”
Dr. Peter Goodwin noted that timescale here is important, and for long term planning, it’s probably more important to look at what’s possible … “If you’re looking at long term infrastructure and community planning, that often takes decades to put in place and make changes to.”
“The earth is out of radiative balance and that’s why we’re warming,” said Dr. Cayan. “As we load the atmosphere with greenhouse gases, as we will continue to do because we have this strong dependence on fossil fuels, we will actually exacerbate the imbalance. Right now there’s about a watt per square meter of imbalance. There’s more energy coming in then is leaving and of course what’s happening is the earth system is warming up. Not just the air temperature but the oceans are warming, every global ocean basin is warming, and partly because of that warming, the water is inflating, it’s expanding thermally. But also because of that warming, we’re releasing the big stockpiles of stored water in glaciers and ice caps, and we’re going to see increasingly the loss of ice in Greenland and Antarctica as decades go forward. This imbalance is not going to be solved over probably our lifetimes and our kids lifetimes and so forth, so sea level rise is going to be a phenomena, we think, for hundreds of years. … It will happen eventually; it’s a matter of when. There are planning horizons and lifetimes of infrastructure projects and so forth, and this all has to be balanced with costs, so that’s what makes your job so interesting.”
“Lest you worry too much about your ability to communicate, your low-key calm clear explanation leaves me the impression that I’ve been read a bedtime story by Vincent Price,” said Councilman Patrick Johnston. [laughter] You’ve scared me sufficiently.”