Global average sea level has risen by about 17 cm between 1900 and 2005. This is a much faster rate than in the previous 3,000 years.
The sea level changes for several reasons, including rising temperatures as fossil fuel burning increases the amount of greenhouse gases in the atmosphere. In a warming climate, the seas are expected to rise at faster rates, increasing the risk of flooding along our coasts. But until now we didn’t know what fraction of the rise was the result of human activities.
In research published in Nature Climate Change, we show for the first time that the burning of fossil fuels is responsible for the majority of sea level rise since the late 20th century.
As the amount of greenhouse gases we are putting into the atmosphere continues to increase, we need to understand how sea level responds. This knowledge can be used to help predict future sea level changes.
Measuring sea level
Nowadays, we can measure the sea surface height using satellites, so we have an accurate idea of how the sea level is changing, both regionally and in the global mean.
Prior to this (before 1993), sea level was measured by tide gauges, which are spread unevenly across the world. As a result, we have a poorer knowledge of how sea level has changed in the past, particularly before 1960 when there were fewer gauges.
Nevertheless, the tide gauge measurements indicate that global mean sea level has increased by about 17 cm between 1900 and 2005.
What drives sea level rise?
The two largest contributors to rising seas are the expansion of the oceans as temperatures rise, loss of mass from glaciers and ice sheets, and other sources of water on land. Although we now know what the most important contributions to sea-level rise are, we did not know what is driving these changes.
Changes in sea level are driven by natural factors such as natural climate variability (for example El Niño), ongoing response to past climate change (regional warming after the Little Ice Age), volcanic eruptions, and changes in the sun’s activity.
Volcanic eruptions and changes in the sun affect sea level across years to decades. Large volcanic eruptions can cause a temporary sea-level fall because the volcanic ash reduces the amount of solar radiation reaching the ocean, thus cooling the ocean.
Humans have also contributed to sea level rise by burning fossil fuels and increasing the concentration of greenhouse gases in the atmosphere.
Separating the causes
We used climate models to estimate ocean expansion and loss of mass from glaciers and ice sheets for each of the individual factors responsible for sea level change (human and natural). To this we added best estimates of all other known contributions to sea level change, such as groundwater extraction and additional ice sheet contributions.
We then compared these model results to the observed global mean 20th century sea-level change to figure out which factor was responsible for a particular amount of sea level change.
Over the 20th century as a whole, the impact of natural influences is small and explains very little of the observed sea-level trend.
The delayed response of the glaciers and ice sheets to the warmer temperatures after the Little Ice Age (1300-1870 AD) caused a sea-level rise in the early 20th century. This explains much of the observed sea-level change before 1950 (almost 70%), but very little after 1970 (less than 10%).
The human factor
The largest contributions to sea-level rise after 1970 are from ocean thermal expansion and the loss of mass from glaciers in response to the warming from increasing greenhouse gas concentrations. This rise is partly offset by the impact of aerosols, which on their own would cause a cooling of the ocean and less melting of glaciers.
The combined influence of these two factors (greenhouse gases and aerosols) is small in the beginning of the century, explaining only about 15% of the observed rise. However, after 1970, we find that the majority of the observed sea-level rise is a direct response to human influence (nearly 70%), with a slightly increasing percentage up to the present day.
When all factors are considered, the models explain about three quarters of the observed rise since 1900 and almost all of the rise over recent decades (almost 90% since 1970).
The reason for this difference can be found either in the models or in the observations. The models could underestimate the observed rise before 1970 due to, for instance, an underestimated ice sheet contribution. However, the quality and number of sea level observations before the satellite altimeter record is also less.
Tipping the scales
Our paper shows that the driving factors of sea-level change have shifted over the course of the 20th century.
Past natural variations in climate were the dominant factor at the start of the century, as a result of glaciers and ice sheets taking decades to centuries to adapt to climate change.
In contrast, by the end of the 20th century, human influence has become the dominant driving factor for sea-level rise. This will probably continue until greenhouse gas emissions are reduced and ocean temperatures, glaciers and ice sheets are in equilibrium with climate again.
About The Author
Aimée Slangen, Postdoctoral research fellow, Institute for Marine and Atmospheric Research
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As cities build more flood-management infrastructure to adapt to the effects of a changing climate, they must go beyond short-term flood protection and consider the long-term effects on the community, its environment, economy, and relationship with the water.
Adapting Cities to Sea Level Rise, by infrastructure expert Stefan Al, introduces design responses to sea-level rise, drawing from examples around the globe. Going against standard engineering solutions, Al argues for approaches that are integrated with the public realm, nature-based, and sensitive to local conditions and the community. He features design responses to building resilience that creates new civic assets for cities. For the first time, the possible infrastructure solutions are brought together in a clear and easy-to-read format.
The first part of the book looks at the challenges for cities that have historically faced sea-level rise and flooding issues, and their response in resiliency through urban design. He presents diverse case studies from New Orleans to Ho Chi Minh to Rotterdam, and draws best practices and urban design typologies for the second part of the book.
Part two is a graphic catalogue of best-practices or resilience strategies. These strategies are organized into four categories: hard protect, soft protect, store, and retreat. The benefits and challenges of each strategy are outlined and highlighted by a case study showing where that strategy has been applied.
Any professional or policymaker in coastal areas seeking to protect their communities from the effects of climate change should start with this book. With the right solutions, Al shows, sea-level rise can become an opportunity to improve our urban areas and landscapes, rather than a threat to our communities.
“Every Floridian should read this book. It is the clearest and most readable description of how and why the sea level changes and what the future has in store for us.”—Orrin H. Pilkey, coauthor of Global Climate Change: A Primer
Sea levels are rising—globally and in Florida. Climatologists, geologists, oceanographers, and the overwhelming majority of the scientific community expect a continuation of this trend for centuries to come. While Florida’s natural history indicates that there is nothing new about the changing elevation of the sea, what is new—and alarming—is the combination of the rising seas and the ever-growing, immobile human infrastructure near the coasts: high-rise condos, suburban developments, tourist meccas, and international metropolises.
The stakes are particularly high in Florida, where much of the landscape is already topographically low and underlain by permeable limestone. Modern-day sea-level rise poses unprecedented challenges for sustainability, urban planning, and political action.
Sea Level Rise in Florida offers an in-depth examination of the rise and fall of sea levels in the past and the science behind the current data, both measured and projected. The authors also discuss ongoing and potential consequences for natural marine and coastal systems and how we can begin to plan strategically for the inevitable changes.
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For as long as humans have been inhabiting coastal areas and recording what occurs in their environments, coastal zones have been defined through dynamic interactions. And this is further underlined by a more recent development: observed sea level rise. In a thorough but not overly technical approach, Adapting to Sea Level Rise in the Coastal Zone: Law and Policy Considerations provides a legal-policy framework for facing the challenges of sea level rise.
The book includes an analysis of sea level rise adaptation strategies that examines the legal impacts of coastal land use decisions based on the current interpretation of private property rights in relation to public control over those rights. The author discusses the science behind sea level rise and highlights policy complexities and options. He then presents an overview of related legalities, and bringing it all together, applies the principles offered in the book, concluding with strategies and solutions and a perspective on the future.
If we accept the premise that sea level rise is occurring and will continue for the foreseeable future, then we must begin to consider policy responses to this risk in coastal regions. Part of any pragmatic policy response must include a review of the options available to public institutions when developing and implementing rational adaptation policies. This book offers practical legal/policy approaches to sea level rise adaptation that promotes sound planning in the face of climate change and rising seas.