Eos, Vol. 92, No. 42, 18 October 2011

PAGE 373
Study suggests Arctic sea ice loss not irreversible
The Arctic has been losing sea ice as Earth’s climate warms, and some studies have suggested that the Arctic could reach a tipping point, beyond which ice would not recover even if global temperatures cooled down again.
However, a new study by Armour et al. that uses a state-of-theart atmosphere-ocean global climate model found no evidence of such irreversibility. In their simulations, the researchers increased atmospheric carbon dioxide levels until Arctic sea ice disappeared year-round and then watched what happened as global temperatures were then decreased. They found that sea ice steadily recovered as global temperatures dropped. An implication of this result is that future sea ice loss will occur only as long as global temperatures continue to

interplanetary magnetic field, sunspot number, and probability of large solar energetic particle events. The researchers found that the risk of space weather radiation events will likely increase noticeably over the next century compared with recent decades and that lower solar activity will lead to increased galactic cosmic ray levels. (Geophysical Research Letters, doi:10.1029/ 2 011GL048489, 2011) —EB

How ocean ridges affect large-­s cale ocean circulation

Northern Hemisphere sea ice area and hemisphericmean temperature, both plotted as 10-year means, when CO2 is increased (red) and then decreased
(blue) in a state-of-the-art climate model.The simulations show no evidence of irreversible sea ice loss.
(Geophysical Research Letters, doi:10.1029/
2011GL048739, 2011) —EB

rise.

Galactic cosmic rays and solar energetic particles can be hazardous to humans in space, damage spacecraft and satellites, pose threats to aircraft electronics, and expose aircrew and passengers to radiation. A new study shows that these threats are likely to increase in coming years as the Sun approaches the end of the period of high solar activity known as “grand solar maximum,” which has persisted through the past several decades. High solar activity can help protect the Earth by repelling incoming galactic cosmic rays.
Understanding the past record can help scientists predict future conditions. Barnard et al. analyzed a 9300-year record of galactic cosmic ray and solar activity based on cosmogenic isotopes in ice cores as well as on neutron monitor data.
They used this to predict future variations in galactic cosmic ray flux, near-Earth

SECCHI-HI instrument team

Predicting space climate change

A coronal mass ejection imaged by the
Solar Terrestrial Relations Observatory (STEREO) A spacecraft about to pass by Mercury and Venus. Barnard et al. show that although such events are likely to become rarer in the future, those that do occur could generate much greater fluxes of hazardous energetic particles. Driving the flow of heat and nutrients around the ocean is the meridional overturning circulation, a large-­s cale current system that in the Atlantic Ocean carries warm surface waters northward and cold, dense water to the south along the bottom of the ocean. An important leg of this journey is the conversion of light warm water into North Atlantic
Deep Water (NADW), which occurs along the coasts of Greenland and
Labrador, Canada. The temperature and density profile of the ocean is important for climate and ecological systems, so understanding any physical features that may impinge on the properties of NADW, such as ocean ridges, is particularly important. Using experiments in a laboratory water tank, Stewart et al. identified how a ridge could affect the circulation patterns in a model ocean. The authors imposed temperature and heat flux gradients on the surface of the ocean analogue and tracked how differences in ridge size, shape, and location affected the temperature and density stratification. They found that when the ridge is sufficiently shallow, such that the depth of the water between the