“We think that this is the most extreme version and it’s not based on facts … It’s not data driven. We’d like to see something that is more data driven. It’s based on modeling, which is extremely hard to do when you’re talking about the climate.”
–White House press secretary, Sarah Huckabee Sanders, speaking at a White House press briefing on Nov. 27 about the Trump Administration’s assessment of the Trump Administration’s recently released climate report
The publicly available report she’s talking about:
12/11/2018
OK, let’s talk about the modeling.
Well, the folks writing the report were asked to make projections about the impacts of climate change on American life, depending on various policy choices we might (or might not) make now. You can’t make a forecast — in any aspect of life — without some sort of modeling. You might forecast how much money you’re going to have at the end of the month. Even if you do this in your head, you will be using something we could call a model. Typically, models involve two main ingredients: (1) relevant data from past experiences, and (2) knowledge or assumptions about how future conditions will likely evolve. You will probably factor into your monetary forecast what you usually spend per week on groceries — that would be (1) data. If it’s December, and you use typical monthly data to make your forecast, you might come up short if you end up spending a lot of money on the Holidays. That would represent a deficiency in part (2), knowledge and assumptions.
Generally, a model is only as good as the data, knowledge, and assumptions on which it’s based, so judging the quality of a model means judging the quality of those.
The recent climate report uses, yes, modeling to project future living conditions in the United States based on various assumptions about policies we might select now. As I pointed out in my last post, this is necessary. If our objective is to ensure we don’t select policies that will result in hundreds of billions of dollars in costs to our economy, the deaths of thousands of Americans, and the displacement of millions more, we can’t just wait and see what happens. That’s because many of the changes caused by climate change, under the worst scenarios, will be irreversible on any time scale of interest to us. If the entire Greenland ice sheet were to melt, for example, sea level would rise an estimated 23 feet. This might make us regretful of the policies we chose which led us to that result. If, learning the error of our ways, we then set about reducing our greenhouse gas emissions, it’s pretty easy to understand the ocean wouldn’t just slither back uphill onto Greenland and re-freeze. The Greenland ice sheet we have now (the one that’s melting) is the result of gradual past snowfall accumulation over millions of years.
So, when Sarah Huckabee Sanders represents that the climate report is not trustworthy because it’s “based on modeling” and “not data driven,” she’s setting up a false choice. We have no alternative but to use modeling! That is, unless the alternative is to bury our heads in the sand and pretend we don’t know anything (which would seem to me outrageously irresponsible). She’s also setting up a false comparison, because the modeling is rigorously derived from past data, as we’ll see below.
And, when she says modeling of the climate is “extremely hard to do,” we should feel gratitude for the wisdom of the many folks who have funded, sponsored, advocated for, and conducted climate research for years — decades!, as summarized (with links to original references) in my Brief History of Climate Change Evidence — who have given us very well developed knowledge and capabilities with respect to climate modeling.
Here’s the crux of the modeling used in Volume II of the climate report. It’s in Chapter 2:
The black lines are data — known fossil fuel emissions (left) and measured temperatures (right). (For details on how the temperature measurements were done, see here.)
The red, blue, and green projections in the left panel are 3 different assumptions about our future carbon emissions, called Relative Concentration Pathways (RCPs). These have been standardized, and their numbers represent the change in radiative forcing in the year 2100 in Watts per square meter of the Earth’s surface. Thus, RCP8.5 (red line) means the “future storyline of fossil fuel emissions” that would result in an additional 8.5 Watts per square meter of energy across the Earth’s surface in the year 2100. Yeah, it’s technical. But, basically, RCP8.5 is the “business as usual scenario.” The one in which we pretend climate change is a hoax, talk about “clean coal” as if burning coal doesn’t produce carbon dioxide (it does, it always does), just go on doing like we’re doing.
The blue and green projections are scenarios in which we act like we give a crap about future humans.
It’s critical to understand that the red, blue, and green lines in the left panel are not yet modeling. They are 3 different assumptions about how we might behave in the future. They represent different policies and actions we might select. For more detailed information about RCPs, see Volume I of the new climate report, Chapter 4.
The modeling appears as the red, blue, and green projections (and the shaded plume associated with each) in the right panel. But, here’s a critical fact. Each one is not just one model. Modeling in the new climate report relies on the Coupled Model Intercomparison Project, Phase 5 (CMIP5). The Coupled Model Intercomparison Project was organized by scientists in 1995 to aggregate and compare the results of the world’s best climate models. Under the administrative leadership of the U.S. Lawrence Livermore National Laboratory, it’s been through 5 refinement phases since, Phase 5 starting in 2010.
If you have a climate model and you want to get it in CMIP5, here’s the price of admission. (1) Your climate model has to mathematically account for physical climate processes in a way that can stand up to scientific scrutiny according to the peer review process. (2) Your climate model needs to fit past historical measurements well. As I touched on in my last post on the new climate report, here are the CMIP5 models (orange cross-hatched band) fitting the past data (black line):
Pretty decent, right?
Here’s how those projections in the top graph are made. Many, many CMIP5 simulations are run. Here’s what that looks like:
On the right panel of the graph at the top, the plumes around the red and green lines are not just artistic renderings of uncertainty. They are the regions within which 95% of the CMIP5 models fall for each scenario.
So, when Sarah says, “It’s not data driven,” I strenuously dispute that claim. It is extremely data driven, in the sense that the climate models being used accurately fit our historical temperature data. And that is a result of decades of data gathering and analytical work refining our ability to understand account for the physical processes that influence global temperature.
When Sarah says, “this is the most extreme version,” I strenuously dispute that claim. The colored plumes around each of the projections in the right panel in the graph at the top represent all of the versions. That’s the range of outcomes predicted by all the world’s climate models! Ok, let’s say we choose “business as usual” (RCP8.5). The “most extreme version” says the average surface temperature of the Earth will increase by almost 10 degrees Fahrenheit by 2100. The least extreme version says the average increase will be around 5 degrees Fahrenheit.
Both of these outcomes, and any of the more probable ones in between, would be terrible! And would be attended by ever more hurricanes, wildfires, droughts, coastal floods, and refugee crises.
…as shown in the rest of the new climate report. More to come.
#rescuethatfrog