By Dr David Whitehouse
BBC News Online science editor
A study of sunlight bounced between the Earth and the Moon shows that during the 80s and 90s the Earth reflected less of our star's light out into space
Sunlight is bounced between the Earth and the Moon
But the trend seems to have been reversed during the past three years.
Researchers think this may be because of the natural variability in cloud cover, which can act to push back the Sun's heat and light away from Earth.
The effect must be taken into account in estimates of future global warming, they report in the journal Science.
The scientists, from the New Jersey Institute of Technology (NJIT) and the California Institute of Technology (Caltech), combined US space agency (Nasa) cloud data from satellites with records of Earth's reflectance off the Moon, called Earthshine.
Earthshine is caused by sunlight reflecting off the Earth and reaching the un-illuminated regions of the Moon's surface.
When this happens, the "dark" side of the Moon glows faintly. It has been called "the old Moon in the new Moon's arms".
"Using a phenomenon first explained by Leonardo da Vinci, we can provide valuable data on the overall reflectance of the Earth and, indirectly, on global cloud cover," says Phil Goode, a physicist at NJIT.
"Our method has the advantage of being very precise, and light reflected by large portions of Earth can be observed simultaneously."
The data shows that the Earth's surface may have been sunnier, or less cloudy, in the 1980s and 1990s, and this would have warmed our planet.
The scientists say that the warming effect of the apparent change in the amount of sunlight reaching the Earth's surface in the two decades is greater than the effect of greenhouse gas warming since 1850.
To look for more recent effects, the research team compared Earthshine measurements from 1999 to mid-2001 with overlapping satellite observations of global cloud properties.
They also used the cloud satellite record for 1983 to 2001 from the Nasa-managed International Satellite Cloud Climatology Project.
By comparing these two records, the team was able to estimate the Earth's albedo - the fraction of light reflected by a body or surface.
The data showed a steady decrease in the Earth's albedo from 1984 to 1995, and then a sharp fall between 1995 and 1996. From 1997 to 2000, the Earth continued to dim.
But from 2001 to 2003 things changed. The Earth brightened to pre-1995 values. The researchers attributed the brightening to global changes in cloud properties.
Cloud cover over an area reflecting light to the Moon during the observations
"At the moment, the cause of these variations is not known, but they imply large shifts in Earth's radiative budget," says Steven Koonin of Caltech.
"Continued observations and modelling efforts will be necessary to learn their implications for climate."
Enric Palle, of NJIT, added: "Our most likely contribution to the global warming debate is to emphasise the role of clouds in climate change must be accounted for, illustrating that we still lack the detailed understanding of our present and past climate system to confidently model future changes."