28 October 2009

Greenhouse gasses cause rise in sea level

How much the sea will rise in the future is fiercely debated and there is also heated discussion over its cause. Both natural phenomena such as volcanic eruptions and solar radiation as well as man’s greenhouse gas emissions have an influence on the climate and in that way on the sea level. Researchers, including those from the Niels Bohr Institute at the University of Copenhagen, have calculated a statistical model of the sea level during the last 1000 years and the results show that during the last 200 years the greatest factors influencing the rise of the sea level were man-made. The results are published in the scientific journal Geophysical Research Letters. 

When the Earth is in a cold period a large part of the Earth’s water is bound up in large ice caps in the polar regions and when the Earth’s climate is warm, a great deal of that ice melts and flows out to sea, which then rises. A warmer climate also means that the sea level rises due to the warmer water, which expands and in that way takes up more space than cold water.

Natural phenomena

Volcanic eruption

Sulphuric acid from volcanic eruptions blocks solar radiation
and as a result cools the earth. Since 1880 volcanic
eruptions have resulted in the sea level being 7 cm lower
than it otherwise would have been. But in that period the sea
level has actually risen 18 cm and that is due primarily to
greenhouse gasses. They are transparent to solar radiation
but block radiation and in that way keep the Earth warm.
Photo: USGS/Cascades Volcano Observatory

There are a number of natural phenomena such as volcanic eruptions and solar radiation, which have a big influence on the climate. Solar radiation has varying cycles due to the Earth’s tilt and orbit around the Sun and this influences the temperature and climate. When there is a violent volcanic eruption large amounts of sulphuric acid are spewed up into the stratosphere where it settles like a blanket, reflecting sunlight so the Earth cools for several years after a giant eruption. 
But man-made changes in the landscape like the cutting down of great swaths of rain forest and agricultural cultivation together with the emission of greenhouse gasses is of great significance for the global climate and sea level. So how does one know whether the rise in temperature and sea level is due to the natural fluctuations of nature or the influence of man?

Anthropogenic (man-made) drivers

”We have used a series of investigations to calculate a model of the sea levels of the last 1000 years compared with the global radiative forcing and brought the results together in conjunction with the natural and man-made drivers”, explains Aslak Grinsted, geophysicist at the Centre for Ice and Climate, Niels Bohr Institute at the University of Copenhagen.

The sea level over the laset 1000 years

On the graph you can see the sea levels of the
last 1000 years. The blue curve shows with the
impact of nature from the sun and volcanic
eruptions on the sea level. The red curve shows
the raised sea level due to the increased man-
made environmental impact. The marked increase
from 1850 is primarily due to the emission of
greenhouse gasses.

The solar activity of the past, volcanic eruptions, as well as the level of greenhouse gasses can be reconstructed using data from ice cores, tree rings, and observations of sun spots. The researchers then investigated the influence of every single source of radiative forcing on sea levels. 

Before the year 1800 it was the natural phenomena like solar radiation and volcanic eruptions that influenced the sea level. However, for the past 200 years the sea level has been controlled primarily by the man-made drivers. The sea has risen approximately 18 cm in the 20th century and of that 4 cm are due to natural changes while 14 cm of the rise in sea level are due first and foremost to greenhouse gasses.

Jevrejeva, S., A. Grinsted, and J. C. Moore (2009), Anthropogenic forcing dominates sea level rise since 1850, Geophys. Res. Lett., doi:10.1029/2009GL040216.

Geophysical Research Letters >>