I am obsessed with “large initial-condition ensembles”—LICE. These are climate models that run multiple scenarios to reflect uncertainties about the system’s past. I’ve been working with them for the last year. Most major climate modeling centers have LICE. They are a valuable scientific tool for trying to understand the relative sizes of a human-caused global warming signal and the noise of natural climate variability.
LICE work like this. You take the same climate model and run it dozens of times, starting from a climate state in 1950 or earlier. In each run, the model is driven by exactly the same external factors—such as human-caused changes in atmospheric levels of greenhouse gases—but starts from slightly different “initial conditions” of the atmosphere and ocean.
One way of varying these initial conditions is by choosing the weather from different days. Another way of scrambling the initial state is by introducing a small random perturbation to the distribution of clouds. Because the climate system is complex and nonlinear, small differences in initial conditions grow over time. Within weeks, the atmosphere has little or no “memory” of its initial state. Within years to a few decades, the ocean, too, “forgets” the initial three-dimension ocean state.
As a result of this chaotic behavior, each individual member of the large ensemble has a different sequence of natural internal climate variability—phenomena like El Niños and La Niñas. This natural climate noise occurs against the backdrop of a slowly-evolving global warming signal driven by greenhouse gas increases. Because these random sequences of internal variability are not correlated across individual runs, averaging over dozens of runs beats down the random noise, yielding a better estimate of the model’s global warming signal. LICE are one of many lines of evidence that the warming since 1950 is large relative to natural climate variability.
In the real world, of course, we don’t have many slightly different parallel Earths or a handy time machine. We can’t travel back to 1950 and change the initial conditions of the climate system. Nor can we travel back to 2016 and change the initial conditions of the U.S. political landscape. We cannot tell the U.S. voters of 2016 that by 2019, students around the world will be striking to protest government inaction on climate change. We cannot tell 2016 voters that some of them will witness the death and destruction caused by hurricanes Harvey, Maria, Florence and Michael. That Paradise will be lost in a firestorm. That President Trump will opt out of the Paris climate accord. That fossil fuel burning will continue unabated, warming the planet, melting major ice sheets, raising sea level, diminishing the habitability of low-lying cities and sowing the seeds of an unprecedented climate-change diaspora.
With the benefit of hindsight, we clearly see consequences of the decision taken by the citizens of the United States on November 8, 2016. We cannot change the decision or the political initial conditions that led to it. We can, however, influence the initial conditions before the next presidential election on November 3, 2020. We can use the intervening 19 months to call out bad behavior.
We can speak out when the president is untruthful, embraces dictators, uses hateful speech, undermines democratic institutions, demeans allies, abrogates treaties and initiates trade wars. We can ask our elected representatives whether their pledge of allegiance is to “one nation, indivisible, with liberty and justice for all” or to Donald J. Trump. We can be tireless advocates for decency, honesty, kindness and rationality. We can ensure that our children’s concerns about their climate future are heard, not mocked. Scientists can speak publicly about the reality and seriousness of human-caused climate change; we can encourage our professional institutions and academies to do the same.
In hindsight, vision is 20/20. While perfect hindsight is helpful in identifying past mistakes and learning from them, a clear view of the choices facing us in 2020 is even more important. Every U.S. citizen needs to understand the possible political and climatic trajectories the world may take after the next presidential election. Some of those trajectories look grim. Others are more hopeful. All of us have influence on the initial conditions.