The earth would have warmed faster in the last two decades had there not been an unexplained rise in the amount of carbon dioxide being absorbed on land, scientists believe.
Scientists have discovered an "abrupt increase'' since 1988 in the uptake of carbon dioxide (CO2) by the land biosphere, which comprises all of the planet's plant and animal ecosystems.
Wellington-based scientist Dr Sara Mikaloff-Fletcher, from the National Institute of Water and Atmospheric Research, was part of the global research team investigating the distribution of CO2 emissions.
Ms Mikaloff-Fletcher said the breakthrough had taken scientists "completely by surprise''.
Although the findings were interesting, she said they created more questions than answers.
"We applied some really exciting statistical techniques ... to look at how (the uptake of CO2 on land) is changing over time.
"We were completely taken by surprise (by the findings). It's opened up a whole new series of questions.''
The findings do not contradict existing science about global warming, but rather explain how much CO2 is absorbed by plants and animals, with some of the CO2 then being passed from plants into the land.
A report into the findings says the increase in uptake is "a big number'', about one billion tonnes of carbon per year.
"To put it into context, that is over 10 per cent of global fossil fuel emissions for 2010,'' the report said.
Applying the new statistical approach to records from as early as the 1950s, the "sharp increase'' in 1988 has continued to the present day.
Ms Mikaloff-Fletcher said there had been some theories about what had caused the increased uptake by the land, but so far no answer has been found.
"What it does mean is that the climate change has been a lot slower than it would have been otherwise (because) less of the CO2 we're producing is staying in the atmosphere.''
Ms Mikaloff-Fletcher said scientists were now trying to establish what caused the increase in CO2 being absorbed by the land and whether that might change in the future.
"While the increase was shown to be significant, the physical processes driving it remain a mystery. It poses big questions for us. What caused this shift? What can it tell us about how land's ability to take up CO2 is going to change in the future? How is that going to feed back into climate conditions in the future?''