AAP FactCheck - Warm air rises, but this does not mean that atmospheric carbon dioxide can't warm the earth's surface, despite claims on social media.

Experts say heat is transferred in all directions, including downwards, via radiation, which is how carbon dioxide (CO2) warms the planet.

They also say this process can be observed using scientific instruments.

The claim is in a Facebook post that claims CO2 cannot be responsible for warming at ground level because "heat only goes up".

"Having reached the air, CO2 no longer has any possible heating effect," the post reads. 

"Have you forgotten what we learned at school, that heat goes UP??"

It goes on to claim that any warming from CO2 would therefore only occur "in the sky". 

"The so-called heating properties of the tiny gas can in no way affect the area of life that we inhabit. Heat does not suddenly come DOWN again, to suit somebody's theory."

However, experts told AAP FactCheck this is incorrect: CO2 molecules transfer heat in all directions, including towards earth's surface.

While warmer air containing CO2 does generally rise above cooler air through convection, atmospheric CO2 warms the planet's surface through a different mechanism: thermal radiation.

Atmospheric CO2 warms the earth's surface primarily through this thermal radiation.

Martin Jucker, a climate physicist at the University of NSW, said that the earth's surface emits heat as longwave radiation, which is absorbed by atmospheric CO2.

The CO2 molecules then re-emit this radiation in all directions, including back towards the surface, preventing it from leaving the atmosphere, like a layer of insulation. 

"CO2 in the atmosphere does not heat the atmosphere, but sends radiation coming from earth's surface back to the surface," Dr Jucker told AAP FactCheck.

"Thus, the energy related to that radiation does not go out to space, but comes back down to earth, heating the surface."

Steven Sherwood, a climate change scientist at the University of NSW, said this downward radiation emitted by atmospheric CO2 can be observed by spectrometers.

"The amount of this heat globally is equivalent to roughly 100 billion blowtorches operating continuously (assuming a blowtorch generates 10kW of heat per unit time)," he told AAP FactCheck.

"It is fully sufficient to explain the observed global temperature rise over time, as verified by calculations."

The mechanisms by which atmospheric CO2 increases global temperature are explained in detail in the most recent IPCC report on the science of climate change.

Dave Frame, a climate change physicist at the University of Canterbury, said the post was mistaken in thinking that direct thermal effects were the only way that things can heat up.

He also said greenhouse gases, such as CO2, trap outgoing longwave electromagnetic radiation and re-emit it toward the planet's surface.

"This is extremely well-established," he told AAP FactCheck.

"A vast number of laboratory, atmospheric, and satellite studies have confirmed this result."

The post also questions evidence of sea level rise, claiming that the only way to measure changes in sea level in all oceans is "to put height detectors in EVERY meter of water".

"I have swum and been boating in many harbours but must confess I have not collided with any of these measuring devices," the post reads. "Nor have I heard of any in any waters around the country. Where are they?"

John Hunter, an expert in sea level rise at the University of Tasmania, said sea levels are measured accurately worldwide.

This is done via a network of instruments, including tide gauges at ports around the world and satellites that can accurately measure ocean heights over hundreds of kilometres.

While there are significant variations in the rate between different locations and over time, sea level is rising in most places and the causes are broadly understood, he said.

"Scientists have put together observations of sea level from land-based and space-based instruments and come up with a consistent picture of the rate of sea-level rise and how it varies from place to place, and over time," he told AAP FactCheck.

Source: AAP FactCheck