Climate Challenges

The Oceans in Crisis

Dr. Stefan Rahmstorf's Report to the German Advisory Council on Global Change

By Hugh S. Galford

German professor and oceanographer Dr. Stefan Rahmstorf, Dean of the Department for Climate Systems at Potsdam, and one of the lead authors of The Future Oceans — Warming Up, Rising High, Turning Sour, presented the report’s major findings at a Washington press conference on March 6, 2007.

Local marine issues such as over-fishing and pollution are now being compounded by the more profound issue of the impact of climate change.

The three main issues facing the oceans, named in the report’s title, are the increasing temperature of ocean waters, the rise of global sea levels and the increased acidification of the seas. He described these as “unprecedented changes — changes that will be irreversible for thousands of years.

“Our generation is causing these changes. It has the responsibility to look at the best science available and at what we’re doing. The question is: is it better to limit emissions now and therefore avoid most of the future attendant risks and damages?”

Politically, he said, “Uncertainty is the main issue. Big, expensive political decisions will be based on these figures.” Both popular and political responses to climate change, he said, are “shifting from ‘It’s not happening’ to ‘We’re done for’. Neither is very helpful — they’re each simply a way to avoid change.” But change, he stressed, is urgently needed. “We can’t change what the world will be in 30 years,” he said, “but our actions in the next decades affect what the world will look like at the turn of the century.”

The European Union, under Germany’s presidency, has made a number of breakthroughs on the issue of climate protection. On March 9, 2007, German Chancellor Angela Merkel announced that the EU’s Heads of States and Governments had agreed to increase the use of renewable energy 20% by 2020. The previous day, the same body had agreed to cut greenhouse gas emissions by 20% from 1990 levels. Both of these binding resolutions came in addition to pledges to increase energy efficiency by 20% and a minimum biofuel content of 10% in fuels.

Such agreements, along with Merkel’s offer to “reduce emissions in Europe by 30% if international partners come on board” at international negotiations, will go some way to protecting the global climate.

Rahmstorf seemed flummoxed by the US’s lack of a sense of urgency on the issue of climate change. He suggested that the White House and politicians look at the Real Climate website ( for the latest information direct from climatologists. In Germany, he said, the situation was different.

“Across the political spectrum there is an agreement that change must occur. The debate is not about the basics, but about what changes are best.”

The Report’s Findings


Since the beginning of the twentieth century, sea-surface temperature has increased 0.6°C. This is lightly lower than the increase in air temperature, as the oceans have a higher heat capacity than landmasses. This rise, however, is only the beginning of greater warming unless we decrease greenhouse gas emissions. The report expects a further 3-5°C rise in sea temperatures this century.

Already fishermen in the North Sea are seeing changes in fish species caught: former species are disappearing, while exotic—that is, southern—fishes are appearing. Ecologically, this could disrupt the food webs in the seas. Economically, the impacts are also fundamental. “These new species,” Rahmstorf said, “are not valuable for fisheries. The overall decline in fish stocks has been due to over-fishing up to now. Climate change is only starting to have an impact, but it will become the ‘800-pound-gorilla’ problem.”

Higher sea temperature also means we should expect hurricanes to grow stronger and bigger, Rahmstorf said. “Hurricane size and strength can be related to sea surface temperature, and the observed temperature increase may extend the range of water at 26.5°C needed for hurricanes to form. Rahmstorf noted that the intensity of hurricanes has increased over the last decades, in step with increases in ocean temperature.


Rahmstorf also noted that sea level has increased 20 cm over the last century. “This is absolutely modern,” he said. “We can gauge where former shorelines were by looking at where the Romans built their seaports.” In the last ten years, half of the observed rise was caused by thermal expansion and half by increased melt-off from ice sheets.

The report predicts rising sea levels ranging from 18 to 59 cm by 2095. This wide range is due to the fact that ice sheet behavior can’t be modeled. For example, we can add 10-20 cm more if we figure in melt-off from the Greenland ice sheet. Given the uncertainties surrounding how the Greenland and West Antarctic ice sheets will respond to temperature pressures, the global mean sea level could be up to 1 meter higher by century’s end.

Rahmstorf pointed out that sea level “responds very slowly, so it is likely to rise by several meters over the next few centuries.” By 2300, the report predicts a 2.5 to 5.1 meter rise over present levels (with global warming limited to 3°C). But Rahmstorf referred back to the difficulty modeling ice sheet behavior: “If the Greenland ice sheet melts entirely, we could add seven meters to sea level forecasts.

“Sea level changes,” Ramstorf said, “will largely be determined by what we humans do in the coming decades. Our generation decides the future of coastal cities.”

Becoming More Acidic:

The third area of the report’s findings, acidification of the oceans, is “not well-known — it is a recent issue in the scientific community.” About 30% of CO2 is held in the oceans, and CO2 concentration is increasing.

In the sea, CO2 is converted into carbonic acid; the pH of near-surface waters has decreased by 0.1 units. The threshold is a 0.2 unit decrease in pH — anything beyond this is a serious stressor for sea life, for at this point the oceans become chemically unsuitable to the formation of calcium carbonate shells, reefs and corals, affecting the bottom of the food chain. Additionally, if corals can’t grow, coastal areas are threatened with greater storm damage and fisheries with the loss of major fish habitats.

Rahmstorf stressed that there was no governmental interference in the reporting process. “There was absolutely no push from any government delegates. German delegates made suggestions on the clarity of presentation, but not on the numbers presented.”

The debate among the scientists was livelier. “Glaciologists,” he said, “thought the numbers and rates presented were potentially underestimated.”

In response to a question about the source of the report’s conclusions, Rahmstorf stressed that they were not drawn solely from computer modeling, which has its limitations. Most of the changes reported, he said, “were based on observational data. We can see the changes, and we understand the underlying basic physics, and so draw our conclusions on that basis.”

In concluding, Rahmstorf brought attention to the report’s “guard rail” concepts — quantitative limits that should be set to minimize environmental damage. Summarized, these guard rails are: to limit air temperature rise to 2°C, to set aside 20 to 30 percent of marine ecosystems as protected areas, to limit absolute sea level rise to 1 meter, and to limit the drop of the oceans’ pH level to 0.2 units. Under these guides, he said, we would be able to sustain the oceans and adapt to the changing conditions.

Cover photo by Wonderlane

Editor's note: This special report, published by the German Advisory Council on Global Change (WBGU), is available online at For this report the Council commissioned four expert studies which are also available at the site, as well as previous studies published since the Council was formed in 1992.

WBGU´s flagship reports provide in-depth scientific explorations of the overarching themes of global change and make recommendations for action and research.

Prof. Dr. Klaus Töpfer, Executive Director of the United Nations Environment Programme (UNEP) from 1998-2006, has called the reports of the German Advisory Council on Global Change (WBGU) 'an indispensable reference and resource on global environmental change policies. Every scientist, decision-maker and institution concerned with the pressing issue of environment and development should have them.'