Human role in climate change is clear

BIG LOSS: Tasman Glacier, from the moraine wall.  Fifteen per cent of the total ice volume of the Southern Alps has been ...
RACHEL HAY

BIG LOSS: Tasman Glacier, from the moraine wall. Fifteen per cent of the total ice volume of the Southern Alps has been lost between 1976 and 2008.

OPINION: 

Warming of the climate system is unequivocal, and human influence on the climate system is clear. These are the conclusions from leading scientists around the world, based on a careful assessment of all the relevant climate research, summarised in recent IPCC reports. These findings are very different from the views expressed by the two authors of an opinion article (Hypothetical global warming: scepticism needed, March 5) in the Dominion Post.

That article overlooks a lot of the relevant science, and misconstrues some of it. We don't expect to change the views of the authors, but we do consider it important that readers are not misled about the factual basis of climate change. Here is some research-based evidence that is relevant to the ten topics raised in the opinion piece.

1. The observed long-term trend in global surface temperature over the past 100 years is clearly upwards. Earth's surface is now on average about 0.85°C warmer than in the late 19th century. Last year - 2014 -was the warmest year globally since comprehensive records began in about 1880.

The long-term warming trend shows intermittent ups and downs due to natural variations in the exchange of heat with the ocean, volcanic eruptions, and fluctuations in energy from the sun. So periods of slower atmospheric warming are expected from time to time, followed by periods of faster warming. These short-term wiggles don't change the long-term picture while greenhouse gas concentrations continue to increase.

2. Arctic sea ice shows a long-term trend of retreat. The average rate of decrease in summer Arctic sea-ice minimum cover since 1979 has been between 9 per cent and 14 per cent per decade - greater in some years, less in others. Antarctic sea ice, which surrounds a huge, isolated and very cold continent, has behaved differently, where some areas have decreased and others increased. Total sea-ice extent (the sum of Arctic and Antarctic) is estimated to have decreased by around 1.5 per cent per decade since 1979.

3. Sea-level has risen by about 19 cm between 1901 and 2010. Natural year-to-year climate fluctuations affect sea-level change over short periods, but the average rate of sea level rise during the past century has been larger than the average during the previous 2000 years.

This makes sense as oceans are warming and hence their water is expanding, and land-based ice is melting. As ice melt accelerates, a faster rise is expected in future.

"Tectonic land movement" may either offset or increase local sea-level change in some places, but earthquakes won't make those risks disappear.

4. Many Arctic ecosystems are already impacted. Effects in recent decades have been observed on permafrost, non-migratory Arctic species, Arctic sea birds, and livelihoods of Arctic indigenous people.

Larger changes are expected in future. Polar scientists dispute the numbers quoted in last week's article for increases in total polar bear populations since 1970. The cited numbers appear to have spread across the internet without a basis in published research data.

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5. Low-lying small islands and global coastlines are at risk from observed and expected further sea level rise. Heat-induced mass coral bleaching and ocean acidification pose further severe future risks for coral reefs.

6. Glaciers have continued to shrink long-term almost worldwide, and Northern Hemisphere spring snow cover has continued to decrease in extent. Again, year-to-year fluctuations and local deviations from the decreasing trend exist, but they don't change the global picture.

7. A substantial contribution to the observed increase in average global surface temperature since the mid-20th century has come from increasing greenhouse gas concentrations. There is no other plausible way to explain the observed changes. This differs from conditions during the "medieval warm period", when some regions were as warm as in the mid-to-late 20th century, but the warm periods did not occur coherently across the globe. For the Northern Hemisphere as a whole, 1983-2012 was likely the warmest 30-year period in at least the last 1400 years.

8. As the oceans are absorbing much of the carbon dioxide added to the atmosphere, they have become more acidic compared to pre-industrial times. There are regional and temporal fluctuations due to natural processes, but the overall global direction is clear and will continue given ongoing carbon dioxide emissions. This poses substantial risks to polar marine ecosystems and to coral reefs, especially under medium-to-high future carbon dioxide emission scenarios.

9. Climate change risks severe yield decreases in major crops (wheat, rice and maize). While carbon dioxide can act as fertiliser, plants also need a supportive climate and soils. The negative effects of increases in temperature (including heat waves) and changes in rainfall and evaporation patterns are expected to outweigh benefits of higher carbon dioxide concentrations for crops in many regions of the world.

10. Changes have been observed in many extreme weather events - and larger changes are expected as the climate continues to warm. These include increases in frequency and duration of heat waves over many land areas, increases in frequency and intensity of heavy rainfall, and more extreme high sea levels.

Human influence on the climate system is clear and growing, and impacts are evident on all continents.

If left unchecked, climate change will increase the likelihood of severe, pervasive and irreversible impacts for people and ecosystems.

We do have options to reduce risks by reducing greenhouse gas emissions and adapting to some climate change, but time is running short if we want to limit changes to manageable levels.

Ignoring or misconstruing the overwhelming evidence is not a responsible risk management strategy.

David Wratt is an Emeritus Climate Scientist at NIWA, an Adjunct Professor in the NZ Climate Change Research Institute at Victoria University, and a Vice Chair of Working Group 1 of the IPCC. Andy Reisinger is Deputy Director (International) of the New Zealand Agricultural Greenhouse Gas Research Centre and served as coordinating lead author in the most recent IPCC report. James Renwick is a Professor of Physical Geography at Victoria University of Wellington and served as a Lead Author on the last two IPCC Reports.

 - The Dominion Post

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