Climate Models Fail To Predict Past Catastrophes
Most people fall into one of two categories when it comes to predictions of future climate calamities: they either do not realize that the predictions are predicated on computer models or they unquestionably trust the models to reveal the future. A clear and lucid online article in Nature Geoscience addresses the current state and limitations of climate modeling. The article points out that State-of-the-art climate models are largely untested against actual occurrences of abrupt change. “It is a huge leap of faith to assume that simulations of the coming century with these models will provide reliable warning of sudden, catastrophic events,” the author states. To counter claims of predicted “tipping points,” incidents of abrupt climate change from the past are examined—incidents that current models get wrong.
As all honest scientists know, predictions by experts of future climatic events are highly subjective. Human intuition fails when confronted with complex, non-linear systems like Earth's climate system. It is reasonable for climate scientists to turn to computer based climate models to provide some insight into how the Earth system might respond to various future changes. “Climate model simulations are the only other means for gaining advance knowledge of sudden climate change,” states Paul Valdes of the School of Geographical Sciences, University of Bristol. “It is therefore crucial to assess whether the available models are capable of investigating these phenomena.”
Motivation for modeling is simple—knowing what our planet's climate will do in the future would be very useful in developmental planning. In particular, the ability to predict sudden, possibly harmful changes in climate could drive policy, save money and even save lives. There have been a number of predictions of catastrophic future events, supposedly based on model output, but the question remains—are they credible? Such changes have undoubtedly occurred in the past. As Valdes states:
Critical thresholds may be inherent to the climate system. If so, they could lead to abrupt, and perhaps irreversible, changes to the Earth system. This possibility has caught the imagination of the public — often under the emotive term 'tipping points' — and has led to a huge growth in media and scientific publications on the topic in the past few years. If we are about to cross such a critical threshold, the implications for climate adaptation strategies could be significant. Likewise, knowledge of thresholds would have a strong influence on mitigation policy, not least by helping to define the meaning of the term 'dangerous climate change'.
In the face of wild predictions of impending climate mayhem what is the public to think? How should such critical thresholds be defined, do they even exist and, if so, are we close to one? Because the climate system is so complex, and direct experimentation is untenable, the author discusses four examples from the past and examines current modeling technology's ability to predict them based on paleodata.
It is hard to identify sudden, dangerous climate excursions from historical data going backs thousands, even millions of years. Nonetheless, scientists know of several dramatic events in the distant past. In “Built for stability,” the four well-documented examples of past rapid climate change and the associated modeling shortcomings are:
- The Paleocene–Eocene Thermal Maximum. A rapid warming event about 55.8 Myr ago started with warm climate conditions with a smaller difference between temperatures at the Equator and the high latitudes. Complex climate models do not adequately simulate the warm climate before the abrupt change set in.
- The desertification of northern Africa. Between about 9,000 and 5,500 yr ago, the region that is now the Sahara was much wetter and supported a steppe-type vegetation. The transition to the current desert state occurred in decades to centuries. Complex climate models fail to simulate the vegetated state, and can not therefore capture this event of rapid change.
- Collapse of the Atlantic meridional overturning circulation. During the glacial period between about 120,000 and 12,000 yr ago, the meriodional overturning circulation in the Atlantic Ocean collapsed during six Heinrich events, most probably in response to fresh water entering the North Atlantic. Complex climate models simulate such a shut-down — but only in response to a freshwater injection as much as ten times the magnitudes estimated for the past.
- Dansgaard–Oeschger rapid warming events. Between Heinrich events, 25 incidences of rapid warming, by up to 8 °C within a few decades in Greenland, are consistently recorded in the ice cores. We don't even fully understand the mechanisms for such changes and simulating the final one of these events required an injection of fresh water into the ocean that was large and many thousand years longer than is thought realistic.
The author examines the Paleocene–Eocene Thermal Maximum in some detail. Around 55.8 million years (Myr) ago, a rapid warming event was accompanied by a significant release of carbon into the environment. In the course of a few thousand years, temperature rose by 5°C in the tropics and by up to 20°C at high latitudes. The event has attracted much interest as an analog for current human GHG emissions though, as I have discussed in previous a post, the analogy is more than a little strained.
But the discussion here is centered on modeling, and according to Dr. Valdes current modeling of the period has a fundamental problem. According to reconstructions, temperatures in the continental interiors rarely dropped below 0°C, even in winter. And geological data suggest that the background climate state of the late Paleocene and early Eocene was characterized by an extremely flat temperature gradient between the Equator and the poles. Problem is, climate models have been unable to simulate the extent of this warming in the higher latitudes.
Simulated and reconstructed early Eocene temperatures.
At high latitudes, the differences between the model and proxy data can exceed 20°C, as shown int the figure above. “Not being able to start from a realistic global temperature distribution for the late Palaeocene makes it unrealistic to simulate the further abrupt warming associated with the Palaeocene–Eocene Thermal Maximum,” states Valdes. “More worryingly, similarly flat latitudinal temperature gradients are a common feature of extreme warm climates of the past, suggesting that IPCC-type, complex climate models may not be well suited to simulating climate dynamics during these past, extremely warm periods.”
The failure of the IPCC models extends to more recent climatic events as well. In the early and mid-Holocene epoch (the current warm period we are enjoying between bouts of glaciation), from around 9,000 and 5,500 yr ago, it was seasonally warmer than today. The region now occupied by the Sahara was much wetter and sufficiently vegetated to be referred to as the 'green Sahara'. Then, about 5,500 yr ago the Sahara went from verdant garden to parched desert rapidly, over decades to centuries.
“Again, the potential relevance to the next century is evident — and again, full-complexity climate models, such as those typically used in the IPCC assessments, do not adequately simulate the climatic conditions before the abrupt change occurred,” Valdes states. The problem here is that the simulations do not generate a green northern Africa during the early and mid-Holocene. Without being able to reproduce the conditions just before the desertification of the Sahara, there is no way the models can simulate its rapid transformation. So we see, it is not just events millions of years ago the models fail to accurately recreate—events that occurred only yesterday, geologically speaking, are also beyond the model's ken.
For the sake of brevity, details on the remaining two examples will be omitted but Valdes' comments on the remaining examples can be found in the online article. For background information on the MOC and Heinrich events see “The Ocean Plays A Deeper Game” and “Modeling Ice Age's End Lessens Climate Change Worries.” For more details on D-O events try here.
In all, this report a blunt assessment of the failings of climate models, and this by a researcher who is a believer in the climate change threat. Regardless, Valdes summarizes his article thus: “I argue that climate models of the current generation, as used in the latest assessment of the Intergovernmental Panel on Climate Change (IPCC), have not proved their ability to simulate abrupt change when a critical threshold is crossed,”
“Overall, the modelling of past abrupt events does not give us confidence in the ability of complex models to simulate critical threshold behaviour that we know has occurred in the past,” he concludes. “If the models are to be used for the prediction of potential future events of abrupt change, their ability to simulate such events needs to be firmly established — science is about evidence, not belief systems.” Well said, but unfortunately, for many climate science is a belief system.
All the nonsense about impending tipping points and catastrophic climate change has either been based on inadequate climate models—shown here to be unable to make such predictions—or has been made up with no foundation at all. The models are not up to the task of predicting such calamities, yet we are constantly told we are near a tipping point or that a tipping point may have already been past. What senseless dribble, what utter baseless tripe. The truth is out, the IPCC models are not capable of predicting future climate catastrophes and only those for whom global warming is a religious belief argue otherwise.
Be safe, enjoy the interglacial and stay skeptical.