Sea-level rise factors unravelled

Myles Gough BBC 13 Apr 16;

Global sea-level rise since the 1970s has been predominantly driven by greenhouse gas emissions and not natural climate variability, a study suggests.

Over the last 100 years, sea levels have been rising much faster than over previous millennia.

Now, scientists have modelled the cumulative forces driving observed sea-level rise in the modern era.

Details of the work are published in Nature Climate Change.

"The influence of greenhouse gases (GHGs) and aerosols - the human component, due to the burning of fossil fuels principally - is small in the beginning of the 20th century, only about 15%," says Dr John Church, a sea-level rise expert at CSIRO, the Australian federal research agency.

"But after 1970 it's the dominant factor, contributing to about 70% of the rise from 1970 up to present day."

"Natural internal climate variability, while it affects sea-level on short periods, has very little impact on the trend during the 20th century."

The findings illustrate the extent to which humans have influenced sea-level rise over the last 115 years, and raise concerns about future sea-level and climate change scenarios.

As the Earth's climate continues to warm, the rate of sea-level rise is expected to accelerate, increasing the risk of coastal flooding.

"There are thresholds we could well cross during the 21st century, which would lead to multi-metre sea-level rise unless there is urgent and significant mitigation," said Dr Church.

If we can stay below a global temperature increase of 2C, Dr Church says the rate of sea-level rise could stabilise, and potentially decrease by the end of the 21st century.

Either way, he says it's imperative that governments begin planning for coastal adaptation, either protecting infrastructure and communities or withdrawing, as some sea-level rise is unavoidable.

Modelling the rise

Using climate model simulations from the latest Intergovernmental Panel on Climate Change (IPCC) report, the CSIRO-led team calculated the main drivers of sea-level rise: glacial and ice sheet melting, and ocean thermal expansion. This is when warmer ocean temperatures cause the volume of water to expand.

The team then applied these calculations to the following individual forces, which contribute to sea-level rise: GHGs, aerosol pollutants in the atmosphere, natural climate variability (such as El Niño and Pacific Decadal Oscillation systems), historical climate responses, variable solar radiation levels, and volcanic eruptions.

After putting them together, and comparing the contributions to observed sea-level rise during the 20th century, the team was able to paint a comprehensive picture of which forces had the biggest impact over that period.

From 1900 to 1950, the biggest factor contributing to sea-level rise was a natural climate response following the Little Ice Age - a period between 1300 and 1870 when the northern hemisphere endured significantly colder winters.

Human influence began increasing around 1950, and became the dominant force after 1970, due to the cumulative effect of greenhouse gases, said Dr Church.

Prof Matthew England, deputy director of the Climate Change Research Centre at UNSW Australia, says the latest findings are "very consistent with what we've long suspected" about the influence of human activities on sea-level rise.

It's consistent with that we know about "how temperatures have warmed globally, and how ice over the land masses of Antarctica and Greenland, have melted over time," he says. "But it's really important to go and do the detailed analysis."

"We can't adapt blind," he says. "We need to understand sea-level rise, how quickly it will change in the future, how it will vary around the coast."