Humans have been driving global warming for nearly two centuries, finds a new study showing climate change isn't just a 20th century phenomenon.
The study, published today in major journal Nature and authored by an international team of 25 scientists, finds that global warming began during the early stages of the industrial revolution and is first detectable in the Arctic and tropical oceans around the 1830s - much earlier than scientists had expected.
The new insights have important implications for assessing the extent that humans have caused the climate to move away from its pre-industrial state, and will help scientists understand the future impact of greenhouse gas emissions on the climate.
The study's lead author, Associate Professor Nerilie Abram from The Australian National University, said anthropogenic (man-made) climate change was generally talked about as a 20th century phenomenon because direct measurements of climate are rare before the 1900s.
However, the team studied detailed reconstructions of climate spanning the past 500 years to identify when the current sustained warming trend really began.
Scientists examined natural records of climate variations across the world's oceans and continents, including climate histories preserved in corals, cave decorations, tree rings and ice cores.
The team also analysed thousands of years of climate model simulations, including experiments used for the latest report by the UN's Intergovernmental Panel on Climate Change (IPCC), to determine what caused the early warming.
The data and simulations pinpointed the early onset of warming to around the 1830s, and found the early warming was attributed to rising greenhouse gas levels.
"It was one of those moments where science really surprised us," Abram said.
"But the results were clear. The climate warming we are witnessing today started about 180 years ago."
Study co-author Dr Helen McGregor, of the University of Wollongong, said humans only caused small increases in the level of greenhouse gases in the atmosphere during the 1800s.
"But the early onset of warming detected in this study indicates the Earth's climate did respond in a rapid and measureable way to even the small increase in carbon emissions during the start of the Industrial Age," she said.
The researchers also studied major volcanic eruptions in the early 1800s and found they were only a minor factor in the early onset of climate warming.
Abram said the earliest signs of greenhouse-induced warming developed during the 1830s in the Arctic and in tropical oceans, followed soon after by Europe, Asia and North America.
However, climate warming appears to have been delayed in the Antarctic, possibly due to the way ocean circulation is pushing warming waters to the North and away from the frozen continent.
Victoria University climate scientist Dr James Renwick described the study as "a very careful and thorough piece of work" demonstrating clearly from observations that humanity has been affecting the climate system for nearly two centuries.
"The climate is very sensitive to atmospheric greenhouse gas levels, especially to carbon dioxide as it stays in the air for centuries," Renwick told the Herald.
"To be able to show that human-induced greenhouse warming is detectable back nearly 200 years is a great piece of detective work, and it complements other results such as a recent finding that a human signature is visible in sea level rise statistics going back more than 150 years.
"It reinforces the importance and urgency of global emissions reductions, since the changes we are making to the atmosphere today will play out in terms of climate changes and sea level rise for decades and centuries to come."
Under present projections, the sea level around New Zealand is expected to rise between 50cm and 100cm this century, leaving populations to adapt by either abandoning coasts and islands, changing infrastructure and coastal zones, or protecting areas with barriers or dykes.
Storms occurring on top of a higher sea level would affect public infrastructure such as roads, railways and stormwater systems, as well as private homes and other buildings.
Climate change was also expected to result in more large storms compounding the effects of sea level rise.
New Zealand - which had a 23 per cent increase in greenhouse gas emissions between 1990 and 2014 - has pledged to slash its greenhouse gas emissions by 30 per cent from 2005 levels and 11 per cent from 1990 levels by 2030.