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up of heat at the Equator. Temperature variations along the boundary drive a large-scale flow that redistributes the excess heat. The large-scale circulation becomes more prominent at low viscosities6, which may explain why the effect had not been noticed in previous calculations that used higher viscosities.

It should be noted that heat flux from the core is not expected to be constant all along the core–mantle boundary, as assumed by the researchers. Fluctuations in mantle temperature are several hundred Kelvin or more, and must cause lateral variations in heat flux from the core. The structure of flow in the core caused by such variations is likely to be more complex than that simulated by Sakura and Roberts6. A far more realistic condition would be a variable heat flux at the top of the core, but at present we lack the information needed to specify this condition.

Previous efforts9 to model the effects of a variable heat flux have relied on variations

in seismic properties of the mantle to constrain the location of temperature anomalies. The resulting flows do resemble those inferred from observed changes in the magnetic field7, but there are important differences between the large-scale magnetic field simulated by such models and the Earth’s magnetic field. In particular, it has been difficult to explain the persistence of high-latitude patches of the Earth’s radial magnetic field. Heat-flux variations at the base of the mantle must have a role, but our understanding of the process is incomplete.

Sakuraba and Roberts6 are not the first to draw attention to the importance of heat-flux conditions in convection problems10. But it had not been appreciated how dramatic the influence of such conditions can be in numerical geodynamo models. Incorporating all the complexities in models is challenging, but the improvements in specific comparisons with the observed magnetic

field as reported in this study are a step in the right direction. ❐

Bruce Buffett is in the Department of Earth and Planetary Science, University of California, Berkeley, 383 McCone Hall, Berkeley, California 94720‑4767, USA. e‑mail: bbuffett@berkeley.edu

References1. Glatzmaier, G. A. & Roberts, P. H. Nature 377, 203–209 (1995).2. Kageyama, A. et al. Phys. Plasmas 2, 1421–1431 (1995).3. Takahashi, F., Matsushima, M. & Honkura, Y. Science

309, 459–461 (2005).4. Christensen, U. R., Olson, P. & Glatzmaier, G. A. Geophys. Res. Lett.

25, 1565–1568 (1998).5. Takahashi, F., Matsushima, M. & Honkura, Y. Phys. Earth Planet.

Inter. 167, 168–178 (2008).6. Sakuraba, A. & Roberts, P. H. Nature Geosci. 2, 802–805 (2009).7. Bloxham, J. & Jackson, A. Geophys. Res. Lett.

17, 1997–2000 (1990).8. Zhang, K. J. Fluid Dyn. 236, 535–556 (1992).9. Olson, P. & Christensen, U. R. Geophys. J. Int. 151, 809–823 (2002).10. Hewitt, J. M., McKenzie, D. P. & Weiss, N. O. Earth Planet. Sci. Lett.

51, 370–380 (1980).11. Jackson, A., Constable, C. & Gillet, N. Geophys. J. Int.

171, 995–1004 (2007).

Human-induced destruction of the stratospheric ozone layer — which lies between 10 and 50 km altitude — was

first detected in the mid 1980s, and has remained a serious environmental concern ever since. Chlorine and bromine account for virtually all of the ozone lost in polar spring, and contribute to ozone depletion at around 20 and 40 km altitude across the globe1. Thus, scientific debate and political action has focused on the influence of chlorine- and bromine-containing chemicals on ozone loss. The Montreal Protocol and its amendments2 therefore led to the phasing out of anthropogenic chlorofluorocarbons and similar halogen-containing chemicals. However, emissions of nitrous oxide — the main source of ozone-destroying nitrogen-based chemicals — remain unregulated by the protocol. Ravishankara and colleagues3 argue that the greenhouse gas nitrous oxide is now the single most important anthropogenic ozone-depleting substance, and stress that emissions reductions of this gas would benefit both the stratospheric ozone layer and climate.

In the 1970s, before chlorine and bromine became the focus of the ozone debate, scientists realized that nitrous oxide was the main source of ozone-depleting nitrogen oxide radicals in the stratosphere. In contrast to chemicals containing chlorine and bromine, nitrogen oxides destroy ozone globally between 25 and 35 km. Nitrous oxide behaves in a similar way to chlorofluorocarbons (CFCs): it is very stable in the lower atmosphere, where it has a lifetime of around 100 years, and is only slowly destroyed by photochemistry after being transported to the stratosphere.

Emissions of nitrous oxide have increased by around 50% since the industrial revolution, and the atmospheric concentration is increasing at a rate of 2–3% per year4. Expansion of agricultural land, and increases in animal production and fertilizer use are driving this increase5. Despite this knowledge, the role of nitrous oxide in human-induced destruction of the stratospheric ozone layer has largely been ignored.

Ravishankara and colleagues3 quantify the ozone depletion potential of nitrous oxide — essentially a measure of the amount of ozone destroyed by nitrous oxide, compared with that destroyed by one of the main CFCs, CFC-11. The ozone depletion potential is used by policymakers to assess the relative impact of different chemicals on stratospheric ozone; it has previously been applied to chlorine- and bromine-containing compounds. Ravishankara and colleagues estimate that the ozone depletion potential of nitrous oxide, under current atmospheric conditions, is 0.017. Although this is around one sixtieth of the ozone depletion potential of CFC-11, it is comparable to many hydrochlorofluorocarbons, which are also being phased out under the Montreal Protocol. By weighting different anthropogenic emissions according to their ozone depletion potential, Ravishankara and colleagues show that nitrous oxide is the most important anthropogenic ozone-depleting substance today. And with anthropogenic emissions set to increase, nitrous oxide is likely to remain the largest

atmospHERic sciEncE

nitrous oxide delays ozone recoveryThe stratospheric ozone layer has undergone severe depletion as a result of anthropogenic halocarbons. Although the Montreal Protocol has provided relief, anthropogenic emissions of another substance, nitrous oxide, are set to dominate ozone destruction.

martyn chipperfield

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source of anthropogenic ozone-depleting chemicals this century6,7.

This discovery provides a strong incentive for regulating nitrous oxide emissions under the Montreal Protocol.

However, nitrous oxide is also a potent greenhouse gas, and thus falls within the remit of the Kyoto Protocol; it recently replaced CFC-12 as the third most important anthropogenic greenhouse gas

after carbon dioxide and methane4. This is but one of many apparent links between the two protocols; for example, the Montreal Protocol has benefited climate through the regulation of CFCs and other ozone-depleting compounds, which also function as greenhouse gases. Clearly there is a need for discussion between the parties to the two protocols, and their successors, regarding nitrous oxide emissions.

The findings of Ravishankara and colleagues3 have re-opened the debate on how best to safeguard the ozone layer. Calculations of nitrous oxide emissions, their attribution to specific sectors, such as different aspects of agriculture and industry, and subsequent atmospheric effects should be further explored in more detailed models. For now, Ravishankara and colleagues have set down a challenge to the scientific community to determine if, how and when controls of anthropogenic nitrous oxide emissions should be implemented. ❐

Martyn Chipperfield is in the School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK. e‑mail: martyn@env.leeds.ac.uk

References1. World Meteorological Organization Scientific Assessment of Ozone

Depletion: 2006 (Global Ozone Research Monitoring Project Report 50, WMO, 2007).

2. The Montreal Protocol on Substances that Deplete the Ozone Layer (United Nations Environment Programme, 1987).

3. Ravishankara, A. R., Daniel, J. S. & Portmann, R. W. Science 326, 123–125 (2009).

4. IPCC Climate Change 2007: The Physical Science Basis (eds Solomon, S. et al.) (Cambridge Univ. Press, 2007).

5. Davidson, E. A. Nature Geosci. 2, 659–662 2009.6. Randeniya, L. K., Vohralik, P. F. & Plumb, I. C. Geophys. Res. Lett.

29, 1051 (2002).7. Chipperfield, M. P. & Feng, W. Geophys. Res. Lett. 30, 1389 (2003).

the extent of summer sea-ice cover in the Arctic region has declined dramatically during the past few

decades. Observational and modelling studies attribute much of the shrinking to atmospheric and oceanic warming1. Indeed, it has been suggested that the Arctic Ocean may become seasonally ice free as early as 2040, because the ice-albedo feedback mechanism promotes a rapid reduction in

sea-ice extent2. The extent of Arctic sea-ice is thought to have implications for heat and moisture exchange in high latitudes and for the circulation in the North Atlantic Ocean2, both in the future and in the past3. On page 772 of this issue, Müller and colleagues4 show that over the past 30,000 years, sea-ice cover in the Fram Strait has varied in concert with climate variability and circulation changes in the North Atlantic Ocean.

By its nature, the extent of sea-ice cover during the past is a difficult parameter to reconstruct. The most direct indication of past sea-ice cover is found in marine sediments, which may include particles entrained and dispersed by sea ice. However, much entrained material will leave no record of its presence until the ice has melted, making the sediment record patchy. Alternatively, there is generally some

palaEocEanoGRapHy

tracking ancient sea iceSea ice is an integral component of the climate system, but a difficult one to reconstruct. Biochemical tracers preserved in marine sediments now reveal the waxing and waning of sea ice since the Last Glacial Maximum in an Arctic Ocean gateway.

niels nørgaard-pedersen

1980 2000 2020 2040 2060

Year

Observations

High emissions scenario

Low emissions scenario

High emissions scenario with fixed nitrous oxide

Depletion due to chlorine and bromine

Stratospheric cooling causesozone ‘super recovery’ enhanced by low nitrous oxide

Recovery from chlorine-and bromine-catalysed loss

Cha

nge

in a

vera

ge o

zone

col

umn

from

1980

(%)

4

2

0

–2

–4

–6

–8

Figure 1 | Influence of greenhouse-gas emissions on stratospheric ozone concentration in northern mid-latitudes. Modelled changes in stratospheric ozone concentration are shown, expressed as a percentage of 1980 values. Projected ozone varies depending on which Intergovernmental Panel on Climate Change (IPCC) greenhouse-gas (carbon dioxide, nitrous oxide, methane) emissions scenario is used4: high emissions scenario (black line); low emissions scenario (red line); or high emissions scenario with nitrous oxide fixed at present day levels (green line). Increasing concentrations of nitrous oxide delay ozone recovery6,7. Observations of past changes are shown in blue. Ravishankara and colleagues3 calculate the ozone depletion potential of nitrous oxide, and show that anthropogenic emissions of nitrous oxide will dominate human-induced ozone destruction this century.

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