PAST AS PROLOGUE
Climate Science Looks Back to Predict the Future—And It’s Not Pretty
Simulating past climates gives science an uncomfortably accurate picture of what the future holds: The last time it was only a little warmer than now, seas were 9 meters higher.
In July, the separation of a giant iceberg from the Larsen-C ice shelf in Antarctica reminded us yet again that we can expect higher sea levels in the future—a scenario that has long been predicted by experts. In fact, higher sea levels are of such interest that climate scientists study warm periods of the past—when sea-levels were higher than today—to predict how our climate will change throughout the 21st century and beyond. The Eemian interglacial period, which occurred around 120,000 years ago, is an example of one such era.
Anyone who doubts the validity of future projections made by climate scientists should pay attention to how well their Earth System Models (ESMs) successfully simulate these past climates. Not only do ESMs do a good job at simulating the climate of today, which is a routine task in the validation of any model poised to make future projections, but past simulations—usually referred to as paleo simulations—have also become remarkably accurate.
During my time as a PhD researcher, I used ESMs to look at ocean temperatures around South Africa in the region where the Agulhas Current flows southwards along the western coast of the continent. Here, the model I used was able to simulate sea surface temperatures during the Last Glacial Maximum, a period around 20,000 years ago at the height of the last ice age, to within a degree of those estimated by geological data.
For me, seeing the results with my own eyes made a difference, and demonstrated the predictive power of these models. Our team also used another similar climate model to look at important weather-affecting changes in Atlantic Ocean circulation during the last ice age (between 80,000 and 20,000 years ago). Again, the modelled data—mostly air temperatures this time—were remarkably accurate when compared to geological data, and the results were published in a paper in Nature.
Notwithstanding the reasonable margins of error which are inherent in both the models and the geological data they are compared with, paleo simulations show us that when you change a model's atmospheric carbon dioxide along with other paleo boundary conditions, the computed results are able to match the past temperatures revealed by geological proxy—and with ever improving accuracy.
Simulating the changing Earth
I can hear you wondering how these past simulations are made. Well, they're made by using the same state-of-the-art ESMs which are routinely used for future Earth prediction, but by applying paleo background conditions. ESMs are built around mathematical descriptions of the physics governing our planet, and have become rather sophisticated in recent years. The models, often referred to as climate models, are built in components, each representing some part of the Earth’s eco-system. Such component parts include the atmosphere, oceans, continental land surface (as well as the vegetation on it), the ice-sheets, and sea-ice. Each component interacts with the others, sending and receiving information about temperature and other parameters which change through time at each location on the modelled planet.
Using these models, climate scientists can create simulations of Earth at different times in the past: from the recent Holocene period, back through the ice ages to the warmer climates of the deeper past, like the Miocene, or deeper still, the Triassic or Cretaceous, when dinosaurs dominated the planet. Super deep time simulations have also been made, including that of “Snowball Earth” a time (or number of times—nobody's sure) when the Earth is thought to have been almost completely covered in ice and snow. The results of paleo simulations are then compared with geological data from ice or sediment cores which are obtained from locations as diverse as ice sheets on Greenland and Antarctica, to African lakes or the ocean floors across the world.
My own ice-age work involved changing a model's atmospheric carbon dioxide level (in that case lowering it to 190 parts per million, from around 400 today), increasing the height and volume of the ice sheets across North America, Europe, and Antarctica, and lowering global sea levels accordingly. The latter effectively closed the Bering Strait, one of the only major ocean gateway differences between the ice age and today. All this is done by coding and reconfiguring the ESM, and by observing the simulated Earth system with a viewer which allows the user to see the planet on an interactive map.
Nowadays, a range of climate models, each built independently of each other, can reconstruct past climate variables like sea surface and air temperature with good accuracy compared to geological data. Again and again, the models produce remarkable results. Even more impressive is that some models are now able to go beyond simply simulating so-called time-slices, i.e., moments in time, say, 20,000 years ago at the height of the last ice age, but are also able to perform transient time varying simulations, say, from the last ice age forward to today, year by year.
On global scales, modelled results are generally respectable compared to geological data. Of course, regional differences are harder to be exact about. And there are other limitations, too. For example, the farther we go back in time, the less accurate the models become. Things get a bit tricky, and with good reason: it's much harder to reconstruct a world millions of years ago than it is one just thousands of years back.
For these so-called deep-time scenarios, there are huge uncertainties in a model's input boundary conditions. Modeling deep time worlds requires complex reconstruction of such things as the period's continental configurations (whether Gondwana or Pangea, or what have you), the associated ocean changes (don't forget oceans have a major control on global climate) and their vegetation types (dense forests at the poles during warmer periods, for example), which all in all involve a lot of unknowns.
It's therefore no surprise that this kind of deep-time modeling is only in its infancy. Having said that, research in the field is ongoing and the future is sure to herald in a new era of discovery.
The Eemian as an analog for the future
Either way, future projections don't involve these continental-scale tectonic changes which are associated with deep time, and are therefore much easier to make. As I've already mentioned, some well constrained past warm periods, such as the Eemian interglacial, stand as good analogs for the climate of the near future, and are used to study potential scenarios.
The Eemian was characterized by temperatures only slightly warmer than today and accompanied by a significantly higher global sea level, between six and nine meters higher. At that time, sea levels had increased to such heights due to the melting ice sheets on Greenland and Antarctica (back then the ice sheets were smaller than today), and also because of the thermal expansion of the ocean caused by higher temperatures. This warmer climate permitted forests to grow at higher latitudes in the northern hemisphere and different species of plants and animals to live in places they would unlikely be found today.
So should we believe that the diversity and geography of our future ecosystems might resemble the Eemian warm period? Well, current observational studies show that significant ecosystem changes are already underway, and modeling suggests that changes in the behavior of many groups of plant and animal species are to be expected. Temperatures are already rising to Eemian levels and the polar ice sheets are indeed melting, which is resulting in sea level rise. But just how close our future resembles the Eemian will depend on our current action on the energy issue, and will ultimately be for future investigators to explore.
Without a doubt, the largest factor determining the Earth's temperature over the next few hundred years is rising atmospheric carbon dioxide. As it stands, our best estimates tell us that by 2100 our global climate will be, on average, anywhere between 2 and 6 degrees warmer than today—enough to cause extreme weather patterns including storms and drought, unknown ocean biodiversity changes, global mass migration, and political chaos.
Most of all, the change will likely usher in the Earth's sixth mass extinction, which is already thought to be underway. This is how humanity will really make its mark on geological time. Perhaps some future human species, or alien race, will one day run paleo simulations of our times and wonder how we managed to cause so much destruction in just a few hundred years.