They said the findings confirm that computer models of climate change were on target and that global temperatures would rise 0.6C (1F) this century, even if greenhouse gases were capped tomorrow.
If carbon dioxide and other heat-trapping emissions instead continue to grow, as expected, things could spin "out of our control", especially as ocean levels rise from melting Greenland and Antarctic ice sheets, the Nasa-led scientists said.
The study, published on Thursday in the journal, Science, is the latest to report growing certainty about global warming projections. A leading European climate scientist called it useful supporting evidence.
More than 1800 instruments-packed floats, deployed in oceans worldwide beginning in 2000, are regularly diving as much as a kilometre undersea to take temperature and other readings.
Their precise measurements are supplemented by better satellite gauging of ocean levels, which rise both from meltwater and as the sea warms and expands.
Researchers, led by Nasa's James Hansen, used the improved data to calculate the oceans' heat content and the global "energy imbalance".
"There can no longer
be doubt that human-made gases are the dominant cause of observed warming"
James Hansen, Nasa's Goddard Institute, Columbia University
They found that for every square metre of surface area, the planet is absorbing almost one watt more of the sun's energy than it is radiating back to space as heat - a historically large imbalance. Such absorbed energy will steadily warm the atmosphere.
The 0.85-watt figure corresponds well with the energy imbalance predicted by the researchers' modelling of climate change through a supercomputer, the report said.
Computer models, numerical simulations of climate change, factor in many influences on climate, including greenhouse
emissions - carbon dioxide, methane and other gases.
Such gases, produced by everything from cars to farms, trap heat as they accumulate in the atmosphere.
Significantly, those emissions have increased at a rate consistent with the detected energy imbalance, the researchers said.
"There can no longer be genuine doubt that human-made gases are the dominant cause of observed warming," said Hansen, director of Nasa's Goddard Institute for Space Studies at Columbia University's Earth Institute.
"This energy imbalance is the 'smoking gun' that we have been looking for."
Fourteen other specialists from Nasa, Columbia and the US Department of Energy co-authored the study. Scientists have found other possible "smoking guns" on global warming in recent years, but Klaus Hasselmann, a leading German climatologist, praised the Hansen report for its innovative work on energy imbalance.
"This is valuable additional supporting evidence" of man-made climate change, he said.
The heat exchange between
Earth and space is out of balance
In February, scientists at San Diego's Scripps Institution of Oceanography said their research - not yet published - also showed a close correlation between climate models and the observed temperatures of oceans, further defusing sceptics' past criticism of uncertainties in modelling.
Average atmospheric temperatures rose about 0.6C in the 20th century, and the Inter-Governmental Panel on Climate Change, a UN-organised network of scientists, says computer modelling shows they will rise between 1.4C and 5.8C by the year 2100, depending on how well emissions are controlled.
The Science study said the excess energy stored in the oceans meant a 0.6C rise in atmospheric temperatures and is already "in the pipeline". This agrees with findings of US government climate modellers, reported last month.
Besides raising ocean levels, global warming is expected to intensify storms, spread disease to new areas and shift climate zones hundreds of kilometres, possibly making farmlands drier and deserts wetter.