21 September 2015 James Hansen and Makiko Sato
S.L. Marcus suggests that our paper “Ice Melt, Sea Level Rise & Superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2°C global warming is highly dangerous” would have greater appeal and impact if it featured some notable, verifiable predictions. In related vein, E. Stabenau asks what observations in the next decade or so would verify our assumptions. Indeed, there are many predictions implicit in our paper, and there is merit in highlighting these. Most revealing, in stark contrast to all IPCC models, is strong cooling of the Southern Ocean surface and in the North Atlantic, as shown in Fig. 1. These coolings are a consequence of fundamental processes induced by injection of meltwater into upper layers of the ocean.
Cooling of the Southern Ocean and North Atlantic results mainly from the stratification effect of freshwater. Lesser density of fresh meltwater, compared to salty ocean water, reduces sinking of surface water to the deep ocean. Reduced Antarctic Bottom Water formation reduces the amount of relatively warm deep water rising to the surface, where it increases heat flux to the atmosphere and space. Instead heat is kept at depth, raising deep water temperature and melting ice shelves (see diagram in Fig. 22 of our paper).
We predict not only that the Southern Ocean surface will cool, rather than warm, but also that the cooling will be largest in the Western Hemisphere. Cooling is larger there because the rate of ice shelf melt is larger there (Fig. 2; note that longitude is shifted 180° in Fig. 2 relative to Fig. 1). Our modeling assumes that warming induced meltwater is three times larger in the Western Hemisphere, stretching from the Ross Sea to the Weddell Sea, than in the other hemisphere. What we have is a push and shove match between global warming, which warms the global ocean surface with amplification at high latitudes, and the freshwater stratification effect, which causes ocean surface cooling in the North Atlantic and Southern Oceans. IPCC simulations for the 21stcentury find a warming Southern Ocean with declining sea ice cover, as freshwater injection is either omitted or small. In contrast, with our assumed rates of freshwater injection, estimated from observations today and extrapolated into the future with several alternative doubling rates, the freshwater cooling effect is already comparable to the greenhouse warming effect in the Southern Ocean, and cooling wins out in our model over the next decade or two. Furthermore, we argue that our model and many ocean models understate the stratification effect because of excessive small scale ocean mixing….We interpret the Southern Ocean cooling and sea ice increase of the past two decades as effects of Antarctic ice shelf melt, i.e., increasing freshwater injection. The sea ice area anomaly decreased sharply in August 2015, back to about the mean value for the base period (1981-2010). We suggest that this sea ice loss is, at least in part, a consequence of the strong 2015-2016 El Nino, which began a few months ago. In other words, in the push and shove match between global warming and freshwater cooling on the Southern Ocean, global warming gets a boost from El Nino, but that boost is temporary. …
….Predictions of ice sheet mass loss and sea level rise. In our paper we discuss potential ice melt doubling times of 10, 20 and 40 years, which respectively would lead to multi-meter sea level rise in about 50, 100, and 200 years. For the sake of analyzing the effect of freshwater on ocean circulation and planetary energy balance, we made climate simulations for doubling times of 5, 10 and 20 years, omitting 40-year doubling because of its larger computing requirement. These cases were sufficient for conclusions about the effect of freshwater on the planetary energy balance and shutdown of overturning ocean circulations (AMOC and SMOC). Here we give our opinion about the likely speed at which ice sheets will respond to the climate forcing for “business-as-usual” growth of fossil fuel emissions. The resulting rate of increasing climate forcing is far outside the rate Earth has ever experienced. We suspect that glaciologists anticipating very slow response of ice sheets base their opinion in part on the rates of ice sheet change that occurred in response to natural climate forcings, which changed much more slowly than the human-made forcing. The rate of change of greenhouse gases determines Earth’s energy imbalance, and the energy imbalance is the “drive” or “forcing” of ice sheet change. …Given all the evidence, a claim that a scenario with 600-900 ppm CO2 forcing within a century would not yield multi-meter sea level rise this century is an extraordinary claim that would require extraordinary proof. Today’s ice sheet models are not capable of providing that proof….Even though a certain “scientific reticence” seems to infect the sea level rise issue, we do not agree that it is already too late to avert climate disasters including: (1) sea level rise inundating coastal cities, (2) shutdown of the Atlantic Meridional Overturning Circulation. Moreover, we will conclude that the actions required to avoid sea level disaster should be sufficient to also avert shutdown of the AMOC and begin to reverse other climate impacts that are beginning to appear and would otherwise be expected to grow under business-as-usual fossil fuel emissions….Let’s discuss actions required to avert climate disasters in the context of responding to a request that we specify needed observations. Critical climate metrics include:
(1) Global surface temperature. Our paper makes clear that the United Nations choice of 2°C as a “guardrail” is not justified by the science, indeed global mean temperature is a flawed metric for that purpose. However, surface temperature is a good diagnostic of the climate system, and, as discussed above, Southern Ocean and North Atlantic temperature patterns will provide an indication of the effect of ice melt on the Southern Ocean and North Atlantic overturning.
(2) Earth’s energy imbalance. The planet’s energy imbalance provides a simple measure of where climate is headed. We must eliminate this imbalance to stabilize climate, and perhaps we will need to achieve a slightly negative imbalance for the purpose of cooling the ocean and avoiding demise of ice shelves and the ice sheets….
(3) Atmospheric CO2. Atmospheric CO2 amount is a critical measure of the state of the planet, which governments apparently prefer to ignore, perhaps because they do not like its implications…. The need to restore Earth’s energy balance informs us about the required limit on greenhouse gases (GHGs), specifically that the CO2 stabilization level cannot be as high as 450 ppm or even 400 ppm, the present amount. Instead it is no more than 350 ppm and possibly lower12, which has immediate implications for policy.
(4) Sea level à ice sheet mass change. Most large cities are located on coast lines. Multi-meter sea level rise has the potential to wreak global economic havoc, create hundreds of millions of refugees, and thus perhaps make the world practically ungovernable….However, large ice sheets are the source of potentially disastrous sea level rise and it is important to measure their rates of change accurately on a regional basis. Thus a critical measurement is continuation of precise gravity measurements from satellites.
(5) Aerosols. Measurements of the largest climate forcings affecting Earth’s energy imbalance are needed for policy prescription. Greenhouse gases are monitored, but the other large human-made forcing, aerosols, including effects on clouds, are not monitored….Analogous to gravity measurements, precise aerosol measurements would be done best from a small satellite, thus making continuous or near -continuous monitoring feasible.
Now let us return to the question: is it already too late? The conclusion that dangerous climate change is reached at global warming less than 2°C, and that it will be necessary to reduce CO2 back below 350 ppm, makes clear how difficult the task will be. The bright side is the fact that the climate forcing limitation required to avoid sea level disaster is so stiff that it should also avert other climate impacts such as AMOC shutdown. Furthermore, we would roll back undesirable climate impacts that are already beginning to appear. There is a misconception that slow feedbacks associated with climate forcings already in place will have unavoidable consequences. Most slow feedbacks will never occur, if we succeed in restoring Earth’s energy balance. Restoration can be aided by reducing non-CO2 forcings. However, the dominance of CO2 in present climate forcing growth, and the long life of fossil fuel carbon in the climate system, demand first attention on phase-out of fossil fuel emissions…..