After three years of drafting and two weeks of virtual negotiations to approve the final draft, the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) confirms that changes are taking place in the Earth’s climate in all continents and oceans.
My contribution was as one of the 15 lead authors of a chapter on the oceans, the world’s frozen landscapes, and sea level change, and this is where we are looking at changes that have become irreversible over centuries, and even millennia.
Overall, the world is now 1.09 ℃ warmer than it was during the 1850-1900 period. The assessment shows that the ocean surface has warmed slightly less, about 0.9 ℃ on a global average, than the land surface since 1850, but about two-thirds of ocean warming has taken place over the past 50 years.
We conclude that it is virtually certain that the ocean’s thermal content will continue to increase for the remainder of the current century, and that it will likely continue until at least 2300, even under low-emission scenarios. Carbon is the main driver of acidification in the open ocean and that acidification has increased faster than at any other time in at least 26,000 years.
We can also say with high confidence that oxygen levels have decreased in many ocean regions since the mid-20th century and that marine heat waves have doubled in frequency since 1980, becoming longer and more intense.
The greenhouse gas emissions of the past, since 1750, mean that we are committed to the future warming of the oceans throughout this century. The pace of change depends on our future emissions, but the process itself is already irreversible on time scales from centenary to millennium.
Loss of ice in Antarctica
All this heat is bad news for the area I work in: Antarctica. With a warming ocean, the Antarctic ice sheet is vulnerable to melting because much of it rests on the bedrock below sea level.
When the ocean warms and the ice sheet melts, sea levels rise around the world. We are convinced that the loss of ice from West Antarctica in recent decades has outpaced the increase in snow mass. We are also confident that this loss was largely due to increased melting of ice below sea level, driven by warming ocean water.
This melting has allowed the acceleration and thinning of the ice on land, and this is what contributes to the rise in sea level. On the other side of the world, the Greenland ice sheet has also lost mass in recent decades, but in Greenland this is mainly due to warmer air, rather than warming ocean water.
It is virtually certain that the melting of the two great ice sheets, Greenland and Antarctica, as well as the many thousands of glaciers around the world, will continue to raise global sea levels for the remainder of the current century.
By 2100, we forecast the global average sea rise to be between 0.4 m (for the lowest emissions scenario, where CO₂ emissions would have to drop to net zero by 2050) and 0.8 m (for the highest emissions scenario) above the 1995-2014 average. The height of the seas in this century clearly depends on how much and how quickly we manage to reduce greenhouse gas emissions.
The time to act is now
There are processes at play that we cannot yet fully capture in computer models, mainly because they take place over longer periods of time than we have from direct (satellite) observations. In Antarctica, some of these uncertain processes could greatly accelerate ice loss and potentially add a meter to the sea level forecast by 2100.
It is not known whether this scenario is true or not, but what is increasingly beyond doubt is that mean sea level will continue to rise for centuries to come. The magnitude of this depends largely on whether we are able to collectively reduce greenhouse gas emissions right now.
The scientific updates to our IE6 chapter are consistent with previous evaluations. This is encouraging, as each evaluation report brings in new authors with different knowledge. The fact that scientific conclusions remain consistent reflects the overwhelming agreement in the global scientific community.
For our chapter, we have evaluated 1,500 research papers, but more than 14,000 publications have been considered throughout IE6, emphasizing recent research that has not been evaluated in previous IPCC reports.
The report has been carefully scrutinized at every stage of its evolution, attracting nearly 80,000 individual peer review comments from around the world. The team of authors has had to respond to each of the comments, with written responses and with careful monitoring of the changes made to the text.
What changes with each assessment is the clarity of the trends we observe and the increasing urgency with which we must act. Although some aspects of IE6 are new, the underlying message remains the same. The longer we wait, the more devastating the consequences will be.
Nick Golledge, Professor of Glaciology, Te Herenga Waka – Victoria University of Wellington
This article was originally published on The Conversation. Read the original.
