We all know that greenhouse gas emissions warm our planet, but we don’t know just how hot, is it going to get? The main reason for this ambiguity is the unpredictability of human. We do not how the people of the late twenty-first century will get their energy or whether they will need as much as we do, or will they adopt fundamentally different lives. This last decision will most likely be made for them by war or societal collapse. All this is not known. Even after removing all the uncertainty associated with politics, economics, technology, and demography, we still can’t be sure as there are many things we still don’t understand about our rapidly warming planet.
To some extent, we even know why we don’t know. Some communication experts sternly claim that “global warming” is a better term for what’s happening than “climate change “while others believe the opposite. But the two events are inseparable as they are feedback reactions of each other. Rising temperatures change the planet that can possibly speed up or even slow down the warming we’ve caused.
Unfortunately, most of these changes worsen our conditions. The melting polar ice is a prime example. Presently, polar ice reflects sunlight back to space thereby cooling the planet like a sunshield on a car windscreen. But after it all melts, the exposed dark land or ocean will absorb rather than reflect the sun making a little bit of warming much more.
We use the artificial but useful concept of climate “sensitivity” to study the effects of these changes. In existing climate models, we presently double atmospheric carbon dioxide from its preindustrial value of 280 parts per million, evolve the model Earth forward to a few hundred years to measure the increase in its temperature. In the first generation of climate models, the recorded increase in temperature varied from about a 1.5 degree Celsius to about 4.5 degrees C. The best estimate was around three degrees C. Even after decades of innovative science and developments in computing power these estimates or their uncertainty has not changed substantially.
We may not know everything, but we do know more than nothing. Every climate model simulates a changing planet in response to variation in temperature. And, increasingly, we understand why they differ on that final warming value. In the climate models that predict higher warming levels, low, thick clouds seem to change in ways that reduce their sun-blocking power. In the models that warm less, these changes are relatively smaller.
So now scientists have focused on measuring clouds, understanding them, and figuring out how to correctly represent them in climate models. Their effort has paid off as the range of uncertainty is now changing. Regrettably, it has increased. Climate models engaging more modern techniques to simulate clouds are now projecting more warming at five – six degrees Celsius in response to a doubling up of carbon dioxide. This increase is substantial as four and a half degrees is the difference between the present day and the last Ice Age.
In the recent past, our Earth is nearly a degree Celsius warmer than it was before the advent of the Industrial Revolution. This is dangerous, but not catastrophic yet, and some experts have suggested that this increase might be indicative of our planet’s relative insensitivity to carbon dioxide. As the past is not the future, we can reasonably assume that there are no analogs for the coming future. At present, heat is getting mixed into the cold, vast and deep ocean, warming it over a period of hundreds or thousands of years. The changes it will subsequently trigger may be different from any we have ever observed. Ice may melt, clouds may dissipate and the warming might get worse. These uncertainties do matter in the real world.
If our climate is very sensitive to carbon dioxide and if the changes triggered by warming themselves contribute to more warming- then the time frame available to us for action is reduced considerably. If our climate is relatively insensitive, then we might have a little more breathing room. But uncertainty is definitely no excuse for inaction. Warming is real and it’s happening. In all likelihood, it’s likely to get worse. Even in the unlikely event that our climate sensitivity is very low, regular human activities attributed to “business as usual” still warm the planet and leading to unpleasant consequences. In the event that our climate sensitivity is high, “business as usual” may spell disaster.
Like all mockups, climate models, are imperfect representations of the real world. They provide useful data about the planet we’re changing, but not exactly how much we’ll change it. The only way to be certain is to truly double atmospheric carbon dioxide and measure the changed while our planet approaches a new equilibrium. Let us hope we never need to conduct such an uncontrolled experiment.