By Earl J. Ritchie, Lecturer, Department of Construction Management
A few years ago, 400 parts per million for carbon dioxide was widely cited as the tipping point for climate change. Now that we have passed that value, it has become common to say that it wasn’t really a tipping point, that it was symbolic or a milestone.
Whether it’s a tipping point or a milestone, we have decisively passed it and CO2 levels appear certain to continue higher. Ralph Keeling, the originator of the famous Keeling Curve, said “it already seems safe to conclude that we won’t be seeing a monthly value below 400 ppm this year – or ever again for the indefinite future.”
Let’s consider what a tipping point actually is. The IPCC describes it as “abrupt and irreversible change.” Lenton, et al. say it “will inevitably lead to a large change of the system, i.e., independently of what might happen to the controls thereafter.” In other words, past the tipping point there will be drastic changes even if we stop emitting CO2. Rather than staying “well below 2 degrees Celsius above pre-industrial levels” as is the target of the United Nations Framework Convention on Climate Change (UNFCCC), there could be warming of several degrees, with associated sea level rise and rainfall changes.
In contrast to these definitions, others say climate change at projected CO2 levels may be reversible. Reversibility is important because otherwise it’s impossible, or at least very difficult, to do anything once you have passed the tipping point. I’ll return to this.
Where do we stand on CO2?
Atmospheric CO2 has not only been increasing; it has been accelerating. The 2001-2016 annual average increase is double that of 1960-1980. As pointed out in an earlier post, commitments under the UNFCC Paris Agreement do not decrease global CO2 emissions, so it is virtually certain that CO2 concentrations will continue to rise.
Much has been made of the potential impact of Trump’s policies on CO2 emissions. The frequently quoted Lux Research analysis of Clinton and Trump policies projected a difference well under a billion metric tons in 2025. This is just over 1% of the world total under the Paris Agreement commitments. The difference is not significant insofar as it relates to tipping mechanisms.
Climate tipping mechanisms
There are multiple possible tipping mechanisms, some of which are shown on the map below. Several of these are occurring today: Arctic sea ice loss, melt of the Greenland ice sheet and boreal forest dieback (and range shifts) are well documented. The extent of permafrost loss, instability of the West Antarctic Ice Sheet and slowing of the Atlantic deep water formation (also called Atlantic Thermohaline Circulation or Atlantic Meridional Overturning Circulation) are less well supported, but there are indications that these are occurring.
These mechanisms are not directly dependent on CO2 concentration; they are triggered by warming alone. Given the amount of warming in recent decades, it is not surprising that they are occurring.
The effects of potential tipping mechanisms are difficult to judge. It’s generally agreed that Arctic sea ice melting is a positive feedback event. Less ice means a darker ocean and more warming. Others are not so clear-cut.
For example, boreal forests, which represent about one-third of the world’s forest cover, are carbon sinks but have variable reflectance depending upon the season, snow cover and vegetation type. Compared to tundra and deciduous forests, they have a net warming effect. The extent to which they will migrate due to warming, and the type of vegetation which will succeed them, are speculative.
Further uncertainty exists because climate effects interact. It is possible to have a cascade, in which increased warming from exceeding one tipping point triggers another.
Is climate change reversible?
The IPCC considers some additional warming irreversible. They say “Many aspects of climate change and associated impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible changes increase as the magnitude of the warming increases.”
Per the models cited in the IPCC assessments, anthropogenic climate change can be halted at 2 degrees, although this scenario requires negative industry and energy-related CO2 emissions later this century. By this interpretation, a tipping point has not been reached.
Accomplishing the 2 degree scenario may be difficult. The world’s track record in emissions reductions is poor. According to Friedlingstein, et al., “Current emission growth rates are twice as large as in the 1990s despite 20 years of international climate negotiations under the United Nations Framework Convention on Climate Change (UNFCCC).”
There has been a reported flattening in fossil fuel emissions for the past couple of years, due primarily to reported coal reductions in China. It remains to be seen whether this is the beginning of a reversal. Even so, emissions would have to decrease rapidly to meet even the 2 degree goal.
Prescriptions for reversal of global warming include proposed geoengineering methods for removing CO2 from the atmosphere and cooling the Earth by reflecting or blocking solar radiation. These do not mean that a tipping point was not passed. In the analogy shown in the cartoon above, one can push the rock back up the hill even after it has rolled to the bottom.
Have we passed the tipping point?
Observed advances in multiple tipping mechanisms certainly raise the question whether the tipping point has been passed. However, these mechanisms are accounted for to at least some degree in climate models, so interpreting that we have passed the tipping point requires that the models understate warming effects.
This is essentially an issue of the sensitivity of climate, that is, how much warming results from a given greenhouse gas concentration. The IPCC’s analysis concludes the likely range of equilibrium sensitivity for doubling of CO2 is 1.5 degrees to 4.5 degrees. As the graph below shows, there is reasonable probability that it could be substantially higher.
If the actual value of sensitivity falls in these higher ranges, warming will be greater than predicted by the IPCC models and a tipping point or points may have been exceeded. I’m not sure that anyone actually knows the answer, which leaves me with the unsatisfactory conclusion of not having answered the question I have raised.
Regardless of whether we have passed the tipping point, continued warming, rainfall pattern changes, significant sea level rise and continued northward and vertical migration of plant and animal species in the Northern Hemisphere seem certain. We are looking at a changed world and must adapt to it.
Not an excuse for inaction
One should not view the possibility that we have passed a significant tipping point as a reason for inaction. Although I remain somewhat skeptical of the degree of human contribution to climate change, it is prudent to take reasonable actions that may reduce the problem. In addition, there are multiple possible tipping points with different thresholds. Exceeding one does not mean you cannot avoid another.
Earl J. Ritchie is a retired energy executive and teaches a course on the oil and gas industry at the University of Houston. He has 35 years’ experience in the industry. He started as a geophysicist with Mobil Oil and subsequently worked in a variety of management and technical positions with several independent exploration and production companies. Ritchie retired as Vice President and General Manager of the offshore division of EOG Resources in 2007. Prior to his experience in the oil industry, he served at the US Air Force Special Weapons Center, providing geologic and geophysical support to nuclear research activities.