When will all the ice in the Arctic be gone? This is a question often asked of sea-ice researchers by the media, the general public and policy makers—and no wonder. Several recent reports have detailed the accelerated loss of summer sea-ice cover in the Arctic. In addition, the observed ice loss is generally happening faster than climate models have forecasted. The question gets even more complicated because we see a large spread in climate model simulations, with ice-free September conditions already happening in 2020 in some simulations but not until well beyond 2100 in others. So determining the answer is tricky. Is this, however, the correct question to be asking in the first place? It assumes sea-ice loss is a function of time, but is that the case?
In reality, Arctic sea-ice cover is not concerned with time. Ice loss is a function of natural climate variability and anthropogenic warming caused by increased atmospheric CO2 concentrations. We monitor summer ice in September because that is the time of year with the least amount of sea ice in the Arctic Ocean. Historically, the Arctic Ocean was covered by ice year-round, but today this area is about half of what it used to be. Over the past three years, several publications have pinpointed warming caused by greenhouse gases as the primary driver of the long-term decline of the summer ice cover. One study for example, showed that for every metric ton of CO2 added to the atmosphere, another three square meters of September sea ice disappear. With current global emission rates of 35 to 40 billion metric tons of CO2 each year, we may get our first glimpses of ice-free Septembers in the next 20 to 25 years, when we will have added another 800 billion metric tons to the atmosphere. Yet it does not stop there. Other months of the year will become ice-free with additional atmospheric CO2. For example, with another 1,800 billion metric tons of CO2, the Arctic will likely have no ice from July through October.
Some researchers are investigating the global temperature limits that will save Arctic sea ice. The probability of losing all the ice is greatly reduced if the warming is kept to below 1.5 degrees Celsius. Ice-free conditions are likely if we allow the warming to increase to two degrees C. The most recent Intergovernmental Panel on Climate Change report gives what is likely a conservative estimate: it states that at increased global temperatures of two degrees C, ice-free Septembers will happen once every decade. The observed relationship between sea-ice decline and global warming is, however, larger than that seen in the climate models. Thus, if we adjust the models to match observations, we find that we lose just more than four million square kilometers of sea ice for each degree C of global warming. Given that current September ice coverage is already hovering near four million square kilometers, this result implies certainty of ice-free conditions each summer at a global warming of 2 degrees C.
Sea-ice loss is not irreversible. So it is time to reframe our question and discuss ways in which we can limit the amount of additional CO2 in the atmosphere in order to preserve summer ice cover in the Arctic. And perhaps more importantly, we must ask how much more sea ice we are willing to lose.