9 January, 2019
“Rising sea level imperils low-lying areas with manifold societal and ecological impacts,” the citation continues. “Because it is influenced by many factors, interpreting and projecting sea-level change is a tremendous scientific challenge.”
Despite being one of the severest impacts of climate change – an increase of just one meter would affect hundreds of millions of people – the extent of sea-level rise is particularly hard for scientists to pin down, due to the large natural variability this magnitude has shown throughout Earth’s history. Some 20,000 years ago – a mere blink on the geological time scale – sea level was 120 meters lower than today’s, and in the last 5,000 years it has climbed by two meters.
Identifying humanity’s footprint within such a wide range has occupied scientists for decades, and it was only two years ago that awardee John Church was able to confirm, writing in Nature, that from 1970 onwards human activity has been the chief factor driving sea-level rise. This conclusion could not have been reached without gathering and integrating data from multiple sources, and it is here that the three laureates’ efforts have been pivotal, on an individual, collaborative and, above all, complementary basis.
Cazenave, Church and Gregory, the committee remarks, “pioneered the integration of satellite observations with in situ measurements and innovations in numerical modeling to develop an accurate and consistent depiction of sea-level rise globally (…) Their findings have been instrumental in testing our understanding of how the Earth system works, enabling better grounded projections.”
Cazenave is Director for Earth Sciences at the International Space Science Institute in Bern, Switzerland, and an emeritus scientist at the Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS), belonging to French space agency CNES. Church is a professor in the University of New South Wales in Sydney (Australia), while Gregory is a professor in the Department of Meteorology at the University of Reading (United Kingdom) and a senior scientist in the National Centre for Atmospheric Science.
A growing rate of increase
Besides identifying the anthropogenic footprint in sea-level rise, laureates’ work has established that the increase is getting faster. “Collectively, their research demonstrated the recent acceleration of regional and global sea-level change and quantified the relative contributions of the different causes of sea-level rise, most importantly ocean thermal expansion and the melting of ice sheets and glaciers driven by anthropogenic global warming,” in the words of the citation.
There is a consensus to the effect that since the early 1990s, sea level has been climbing at a rate of 3 millimeters a year, giving a mean increase of 8 centimeters over the last 25 years. And that is a lot. The latest report from the Intergovernmental Panel on Climate Change (IPCC), published in 2014, states that “the rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia.” And to make matters worse, observations show that the pace picked up over the 20th century, and will very likely continue to do so.
Observations from space
Satellite observations have played a crucial role in facilitating these conclusions. Initiated in the 1990s, their global, planetary perspective was able to impose order on the tide records gathered haphazardly for decades at ports and other points along the coasts. Cazenave, a geophysics specialist, was a pioneer in interpreting these new measurements, whose first source was the Franco-American satellite TOPEX/Poseidon. Her work corrected the errors of previous estimates to deliver the first accurate and reliable set of data on sea level globally.
As she explains, “observations from space have had a key role. Before the satellite altimetry era that began at the start of the 1990s, the only information we had about sea-level rise was from an instrument installed in ports, which gave no data on the open ocean. So very little was known about the phenomenon.”
Church, an oceanographer, and Cazenave were able to reconcile the new satellite data with existing local registers, and by this means build a solid record of sea-level change in the recent period. The next steps were to extend their records into the past and draw up projections for the future, and it was here that Gregory’s numerical models came into their own.
This British researcher is an expert in calibrating climate sensitivity to diverse factors, be it the rise in atmospheric CO2 or the speed of ice melt. Thanks to his work, such variables can now be integrated and existing data adjusted in a way that permits reliable predictions of the system’s future evolution, with a known margin of uncertainty.
“What I have done mostly is work on how to put together models that include all components of sea-level change, in order to make projections for the future and to improve understanding of the past,” he explains.
We now possess a robust temporal record of sea levels from the end of the 18th to the 21st century. Moreover, in the few years separating one IPCC report from the next, prediction uncertainty has lessened substantially. “Our confidence has improved because we have gained a greater understanding of the past, such that we can now explain how and why sea level has changed over the past 150 years,” says Gregory. “A better understanding of the past gives us more confidence in the future.”
Adapt as a matter of urgency
The three laureates coincide regarding the gravity and urgency of the problem, and the need for a political response. “Sea level is rising at an increasing rate,” says Church. “If it is business as usual, and we fail to curb our emissions, we could see a sea-level rise of up to a meter, perhaps more by the end of the century. But with urgent and significant mitigation, we could reduce that rise to maybe a little over half a meter. The rise in any case will be ongoing for many centuries.”
Church also pointed out the vast numbers of people at risk: “About 100 million people live within one meter of current high tide level, so in a century we could have 100 million people having to adapt in some way, either protecting their houses, retreating from the coastline or protecting the coastline.”
“This problem is not just important, it’s urgent,” Church insists. “Many politicians probably recognize the importance but few recognize how urgent a response is required (…) Reducing emissions to achieve a temperature increase no higher than 1.5 degrees is a good target; it is a difficult task but that is what we should be aiming for.”
“In Spain, and any other country with a coastline, we need to plan for rising sea levels during the 21st century and beyond. Because sea-level rise will not stop in 2100. And of course we have to mitigate emissions in order to avoid the worst-case scenarios.”
“It is not too late”
Sea-level rise may be inevitable, but this should not lead us to inaction, warns Gregory: “Even in the best emission reduction scenario, sea-level rise is not stabilized at the end of the 21st century. It will proceed for many centuries to come, because the time scale for the warming of the deep ocean is centuries or millennia. However, we can have an influence on how much and how fast it will happen. We can’t stop the increase, but we are not too late to do something to mitigate it and reduce its impact.”
To those who ignore the findings of climate change science, his message is blunt: “These are facts; it is not a matter of opinion. Science deals with objective facts and you can’t set aside an objective fact because you don’t like it.”
Cavenaze too calls for further action: “It is clear that we are not doing enough to combat climate change. Sea-level rise is just one consequence of global warming. There are many others, like extreme weather events, that are becoming more intense. We have to change the model of our society and way of life, consuming less fossil fuels. In Europe the general public is aware of the problem, but at government level we are still waiting for concrete action.”
“To skeptics I would say compare the current climate in 2019 to that of 10 or 15 years ago. I would simply say to them to look at the facts and observations,” she concludes.