The planet is already committed to at least 0.6 degrees Celsius of global warming by the end of this century because of heat stored in its oceans, say scientists.
The stored energy is solar energy that was trapped by greenhouse gases in the Earth's atmosphere, they say, adding that there is a time lag between when the planet receives the energy and when its effects are seen.
They warn that this means that delaying action on climate change until its effects are already apparent would commit the planet to still greater climate change that would be "difficult or impossible to avoid".
In a study published online last week (29 April) by the journal Science, the US-based climatologists showed that Earth is absorbing more energy from the Sun than it is sending back out into space.
The team led by James Hansen, director of the NASA Goddard Institute for Space Studies in New York, United States, say this is throwing the Earth's energy "out of balance" and causing the planet to heat up.
The consequences, including melting polar ice, suggest our climate system might already be out of our control, say the authors.
They say the Earth's warming is probably caused by human activities, particularly emissions of carbon dioxide, methane and soot into the atmosphere. These trap heat from the Sun and stop it from escaping into space.
"This energy imbalance is the 'smoking gun' that we have been looking for," says Hansen, adding that his team's research shows that estimates of human and natural factors affecting climate "are about right, and they are driving the Earth toward a warmer climate".
Most of the trapped energy is stored in the oceans, which release heat more slowly than the land.
The scientists say this time lag means there is already 0.6 degrees Celsius of global warming "in the pipeline" — an unavoidable result of past emissions.
Based on sea levels during previous periods of warmer climates on Earth, climatologists have suggested that one degree Celsius above temperatures recorded in 2000 would constitute dangerous interference with the climate.
But these levels might be difficult to avoid, say Hansen's team because of the 'inevitable' 0.6 degrees Celsius of warming and "a still expanding worldwide fossil fuel energy infrastructure".
As the planet warms up, ice sheets at the Earth's poles melt, leading to rising sea levels.
The researchers estimate that sea levels have already risen by about 1.5 centimetres in the past ten years because of melting ice sheets. Yet this increase used just two per cent of the energy stored in the oceans.
They conclude that we could already be faced with a climate system that is "out of our control".
Because of the delayed release of heat from the oceans, action taken now could reduce the impacts of climate change.
However, if we wait to accumulate evidence of climate change, say the scientists, the 'stored' global warming could mean that worse climate change will be inevitable.