Early human activity linked to sea-level rise
5 October 2009, by Tamera Jones
Researchers have discovered that human activity is to blame for more than half the changes in sea level since 1850.
And during the twentieth century, only four centimetres of the observed 18 centimetre sea-level rise is down to natural variation. The remaining 14 centimetres is a direct result of human activities.
They also found that before 1800 any change in sea level can be explained by natural variation, caused by phenomena like changes in the amount of heat reaching the Earth from the Sun or the cooling effects of volcanic eruptions.
The researchers report that a large part of sea-level rise after 1800 is a result of an increase in atmospheric carbon dioxide (CO2) from deforestation and then later, burning fossil fuels from the start of the Industrial Revolution.
Shocking
'It's shocking that so much of the sea-level rise we saw in the twentieth century is because of what we're doing. What's surprising is if we hadn't put carbon dioxide (CO2) into the atmosphere, sea level would have risen by only four centimetres last century,' says Dr Svetlana Jevrejeva from the Proudman Oceanographic Laboratory in Liverpool.
Earlier research on the impact of climate change has tended to focus on the effect of one variable, such as a change in temperature.
'There's a century-scale time-delay because it takes time for the oceans to warm and ice to melt or glaciers to grow.'
Dr Svetlana Jevrejeva, Proudman Oceanographic Laboratory
But Jevrejeva's team used a complex statistical model to study the effects of various natural factors and human activities over the past 1000 years on sea-level rise. They compared their results with measurements of sea level from tide gauge records since 1700.
In the model, they assumed that sea level rise is caused mainly by melting glaciers and ice sheets and changes in the heat content of the world's oceans (when water heats up, it expands). Natural factors, such as volcanic eruptions, and human activities that cause climate change affect both of these.
Jevrejeva was initially only interested in finding out how much sea level rose over the last century.
Changes in sea level are essentially caused by changes in global temperature. These changes in temperature are caused by volcanic eruptions, variations in the amount of solar radiation reaching the Earth from the Sun or greenhouse gases accumulating in the atmosphere.
Volcanic particles act like clouds and stop sunlight reaching the Earth, so the planet ends up cooling down. But when the amount of solar radiation reaching the Earth from the Sun rises, or greenhouse gases collect in the atmosphere, the planet warms up.
'There's a century-scale time-delay because it takes time for the oceans to warm and ice to melt or glaciers to grow,' explains Jevrejeva.
Jevrejeva and her colleagues report in the journal Geophysical Research Letters that for the last 200 years, sea-level rise has mostly been driven by anthropogenic factors.
This contrasts with the latest findings by the Intergovernmental Panel on Climate Change which reported that sea levels rose because of human activities only after 1950.
The team also reports that things could've been a lot worse if it hadn't been for volcanic eruptions. 'If no volcanic eruptions had happened since 1880, then sea-level rise over the last century would've been seven centimetres higher than it was,' explains Jevrejeva.
She adds: 'It's possible that volcanic eruptions have masked a contribution from greenhouse gases (from land-use change) since as early as 1700.'
Jevrejeva, S., A. Grinsted, and J. C. Moore (2009),
Anthropogenic forcing dominates sea level rise since 1850,
Geophys. Res. Lett., doi:10.1029/2009GL040216, in press.
(accepted 23 September 2009)
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