By Jonathan Amos
Science reporter, BBC News, San Francisco
Even a small-scale nuclear war would have far-reaching consequences for the global climate, say scientists.
Hiroshima's Atomic Dome: A warning to the world
A US team has modelled the effects of a limited conflict in the light of new concerns over weapons proliferation.
Its study, reported at an American Geophysical Union meeting, employed the very latest computer simulations.
For an exchange of 100 Hiroshima-sized bombs, the modelling suggested there would be millions of deaths, as well as climate cooling and ozone damage.
The team said increasing urbanisation and the centralisation of economies in megacities had made modern society more vulnerable to the destabilising effects of war.
"We saw what happened on 9/11: 3,000 people died and the world economy staggered," said Professor Richard Turco from the University of California, Los Angeles (UCLA).
"One can only imagine the implications if Hiroshima-scale detonations occurred in one or more major cities around the world today."
It was Professor Turco who first coined the phrase "nuclear winter" back in the 1980s to describe the climate shift that would result from a war using atomic weapons.
The latest investigation is written up in two papers in the journal Atmospheric Chemistry and Physics Discussions.
For 50 15kt airburst detonations centred on urban zones; blast and fire deaths
It draws on expertise from UCLA, Rutgers University, New Jersey, and the University of Colorado at Boulder.
The team assumed an engagement between two emerging countries in the subtropics, and that the attacks with 15-kiloton devices were focused on large metropolitan centres.
The explosions would ignite huge firestorms, fuelled by all the wood, plastics and petroleum products commonly found in such locations.
These would send thick black smoke into the upper troposphere, where heating by the Sun would drive the particles even higher into the stratosphere.
The new simulations, run on a Nasa supercomputer, show that for five million tonnes of black smoke sent skyward, the result would be a global cooling of 1.25C, as the material spread out and blocked light from reaching the ground.
A reduction in precipitation levels of 10% and a shortening of growing seasons by up to 30 days was also forecast.
The impact on global food supplies would be catastrophic, the group said.
The researchers warned that the impoverished conditions would last longer than previous studies had indicated because older climate models did not adequately represent the mechanisms that took material high into the stratosphere.
"If you look at the change in solar radiation from, say, the Mount Pinatubo volcanic eruption (1991), it goes away after about a year, but the black smoke is lofted into the stratosphere and it lasts for a decade," explained Professor Alan Robock from Rutgers.
"The effects last much longer than we thought previously because we now have the computer power and the detailed models to calculate the entire depth of the atmosphere and simulate that."
Some regions would probably go back to cold conditions not experienced since medieval times and a climate phase sometimes referred to as the "Little Ice Age" when Europe in particular was hit by very harsh winters.
The authors also note that the smoke plumes would upset stratospheric chemistry, resulting in a significant disruption of the ozone layer, depleting it by 40% over many inhabited areas and by up to 70-80% at the poles.
The researchers said current concern about emerging nations acquiring nuclear weapons and the threat of terrorist attacks merited a return to the subject.
"While most people think we're on a path of reduced probability of war because of nuclear build-down by the superpowers, in fact we actually have the opposite situation going on," said Professor Owen Toon from Colorado University.
"We're on a trend of a build-up of nuclear weapons. In a couple of decades, there could be 20, 30, 40 nuclear states all as dangerous as the Soviet Union used to be."