Negative-Emission Whales

Whales are big. Like all biomass, their bodies sequester carbon from the biosphere. So it seems like if we had more whales in the oceans, they could capture a decent amount of carbon emissions just by existing and living their lives. A team of economists at the International Monetary Fund recently set out to quantify that amount, and they came up with an impressive figure.

According to the analysis, between the carbon in dead whale bodies and the boosted ocean life productivity created by their waste, whales could cumulatively sequester 1.7 billion tons of CO2 every year if their populations returned to pre-whaling levels — roughly 5% of global emissions.

The notion of whales as carbon sinks isn’t radically different from some other, more prominently proposed “natural” solutions to climate change. Much has been made in recent years, for instance, of negative-emission cows, and the potential to harvest massive production forests for bioenergy with carbon capture and storage (BECCS).

The problem with such so-called natural solutions, however, is that they suffer from significant scientific uncertainties, throwing their carbon mitigation potential into question.

Consider the whales. In the IMF analysis, Ralph Chami and his co-authors arrive at a figure suggesting that the pre-industrial large whale population of 4-5 million individuals sequestered around 1.7 billion tons of CO2 annually. This implies a carbon storage potential of 340-425 tons of CO2 per whale.

More rigorous scientific estimates peg the figure closer to 50-60 tons of CO2 per Southern Ocean sperm whale. But even these studies likely substantially overstate whales’ contribution to carbon storage. Estimates of the carbon stored by great whales are complicated by numerous uncertainties, ranging from the efficiency of waste-borne nutrient release to the ratio of adult to juvenile whales. Seasonal changes in whale diet and differences in feeding behavior across whale species in particular prevent direct extrapolation of sperm whale–driven carbon sequestration figures to other species. Many whale species both feed and excrete nearer to the surface, for example, a behavior pattern that is thought to have more of a net-zero effect on carbon sequestration.

Most importantly, the efficiency with which nutrients in whale excrement are converted to carbon storage is poorly constrained, with estimates of how much carbon is sequestered per additional unit of iron (the key nutrient provided by whale feces) varying by multiple orders of magnitude. Each ecosystem itself features a different sequestration efficiency that depends on the time of year, the composition of organisms, and the availability of nutrients. Whether a humpback whale lives off the coast of Hawaii, California, or Alaska may significantly affect the magnitude of its carbon impact.

In short, considering this complex backdrop of nutrient ratios, whale biology, and marine ecology, touting the economic value of breeding and maintaining whale populations is neither a reasonable carbon sequestration strategy nor a well-founded monetary valuation.

It’s convenient to assume that vast amounts of anthropogenic emissions can be sequestered in forests, soils, or whales, but nature is not always so accommodating. Forests, for instance, can (and do) sequester significant stores of carbon dioxide. But different climates, changes in ecosystem respiration, fires, and other factors can significantly shift the degree to which different forest ecosystems sequester carbon over time. Similarly, improved soil management (for instance, in farming and ranching) can certainly increase the sequestration potential of croplands and grazing lands. But serious uncertainties remain over the longevity of carbon sequestration, saturation rates, and, indeed, how to come close to quantifying actual carbon stored in soils. The ability of oceans to sequester carbon is reliant on rapidly-responding, unpredictable algae and microorganisms, and that comes with even bigger question marks.

An effective climate strategy will include a lot of carbon sequestration. Natural climate solutions — modern forestry practices, soil management, and boosting whale populations — will likely result in some estimable carbon reduction. But to meaningfully bend the warming curve at the scale we need, those kinds of solutions don’t obviate the need for engineered systems, like carbon capture at power plants, direct-air capture (DAC), and agricultural intensification.

So next time you hear impressive-sounding claims about managing soils, forests, or whales as a large-scale carbon capture solution, be sure to look at the fine print.