Greenland ice cores have revealed that changes in Greenland surface temperature and atmospheric methane emissions at the onset of a rapid warming period of about two centuries during the last deglaciation occurred essentially synchronously to within at most a couple of decades of each other. It was concluded that to find Greenland surface temperature and atmospheric methane rising in such close step is a strong indication that this rapid warming event was not localized to Greenland or high latitudes but was hemispheric including the tropics.
The last glaciation (110,000 to 12,000 years ago) was punctuated by rapid warming/cooling events of 1500 years in duration that occurred 25 times. Comparison of evidence from Greenland and Antarctic ice cores suggests that the Northern Hemisphere warming events are related via a see-saw to cool periods in Antarctica and vice versa. The warm periods in the south corresponding the northern cool periods are called Antarctic Isotope Maxima or AIM.
The Northern Hemisphere warming can occur as quickly as decades, the cooling is slower occurring over centuries. The best evidence for these warming events is found in the Greenland ice cores, which go back to the end of the last interglacial period about 110,000 years ago.
In this study air bubbles in Greenland ice cores are analyzed to compare changes in Greenland surface temperature and atmospheric methane concentration during a short warming event during the last deglaciation. Methane concentration and a proxy for Greenland surface temperature was measured for a period of rapid warming lasting about two centuries and which started 12,800 years before the present just before the current warm period. This abrupt warming in Greenland amounted to about 8 °C over 200 years and initiated the Bølling–Allerød warm period which was the last of the 1500 year long warming/cooling periods during the last ice age. Since the source of methane emissions is typically tropical and boreal wetlands which are far from Greenland, methane concentrations is treated as an indicator of the global climate in low latitudes.
Linking temperature variability over Greenland and in the tropics
Around 14,500 years ago, a an abrupt warming led to the warm period known as the Bølling–Allerød. This period has been seen in proxy records including Greenland ice cores. It was followed by a 1,300 year period of cold climatic conditions and drought which occurred between approximately 12,800 and 11,500 years before the present (BP). This cooling period, referred to as the Younger Dryas, was followed by the warming which led to the present interglacial. Despite the evidence for the Bølling–Allerød warming in many palaeoclimate records, the relative timing of changes in temperature and atmospheric greenhouse gases in different regions of the world remains an open question.
Methane (CH4) is an important greenhouse gas emitted from wetlands. Methane sources are presumed to be boreal or tropical. The largest methane source is the tropics – the authors estimate that during the Bølling–Allerød warming boreal sources could contribute only about 22% of the total methane increase. Because methane is quickly mixed through the atmosphere, methane concentrations in air bubbles can be used to link Greenland and the tropics.
The phasing of global climate change at the onset of the Bølling–Allerød event were studied using air preserved in bubbles in the North Greenland Eemian (NEEM) ice core. Specifically, methane concentrations, which act as a proxy for low-latitude climate, and the nitrogen-15/nitrogen-14 ratio of atmospheric nitrogen, which reflects Greenland surface temperature, were measured over the same interval of time.
Gases become trapped in bubbles at the bottom of the layer of increasingly dense snow known as the firn which ultimately becomes ice. Air bubbles are captured in the firn at a depth of 60–100 meters. There are several factors that complicate the relative chronology of temperature and greenhouse gas concentrations in ice cores. First of all, air can diffuse up to 60-100 meters in the firn (the layer where snow is gradually compacted into ice) so that the air bubbles captured at any given level in the ice do not correspond to the age of the surrounding ice. Another problem is that methane diffuses faster in the firn than nitrogen. In addition a high resolution analysis needs to account for methane mixing in the atmosphere over a large geographic area relative to its atmospheric lifetime of about 10 years.
In this study the authors partly avoid the first problem by using nitrogen isotope ratio (ratio of nitrogen-15 to nitrogen-14) in the air bubbles themselves instead of isotopes in the surrounding ice as a temperature proxy. The nitrogen-15/nitrogen-14 ratio of atmospheric nitrogen provides a thermometer for abrupt changes in surface temperature at an ice core site.
To account for the other problems: atmospheric mixing and the lifetime of methane smoothing the impact of an abrupt increase in methane emissions on atmospheric methane concentration over Greenland and methane moving through the firn more quickly than N2 because of its greater diffusivity, the authors augment the observed data with the results of modeling. They use a simple atmospheric model to account for methane mixing and sinks in the atmosphere. They use a firn model to take into account the relative rates of diffusion of nitrogen and methane in the firn.
The initial increase in both gases starts at 14,698 years before the present (years BP) with a relative uncertainty of 5 years. The increases are easy to detect because the first data point of each increase is much larger than the uncertainty in the previous data points. (The magnitude of the increase exceeds the uncertainty of the measurement by a factor of 5 for methane concentration and 8 for nitrogen-15/nitrogen-14 ratio.)
The results show that methane emissions and Greenland temperature changed essentially synchronously to within decades. Within the estimated errors it was not possible to determine whether one led the other.
Methane could lead temperature
The uncertainty estimate suggest that methane sources could lead Greenland temperature by up to 24 years. This is in agreement with the conclusions of Steffensen, J. P. et al. High-resolution Greenland ice core data show abrupt climate change happens in few years. Science 321, 680–684 (2008), which found a 10 year( ± 5 years) year lead of tropical regions over high-latitude sites by analyzing the North Greenland Ice Core Project (NGRIP) ice core.
Temperature could lead methane
Alternatively, methane could lag temperature by up to 21 years. This would be partially in agreement with the previous estimates of Severinghaus, J. & Brook, E. Abrupt climate change at the end of the last glacial period inferred from trapped air in polar ice. Science 286, 930–934 (1999) based on lower-resolution and lower-precision ice core gas data from the Greenland GISP2 ice cores in which a 20–80 year lag of tropical methane emissions behind the temperature proxy was found for the Bølling–Allerød warming event.
The research found that changes in Greenland surface temperature and atmospheric methane emissions at the onset of the warming period occurred essentially synchronously to within at most a couple of decades of each other. The uncertainties in estimating methane mixing in the atmosphere and diffusion of gases in the firn complicate the interpretation of very high-resolution ice core data. This introduces uncertainty into the interpretation ice core gas records at time intervals less than decades. It was concluded that to find Greenland surface temperature and atmospheric methane rising in such close step is a strong indication that this rapid warming event was not localized to Greenland but occurred over the entire hemisphere including the tropics.
An ice core record of near-synchronous global climate changes at the Bølling transition, Julia L. Rosen, Edward J. Brook, Jeffrey P. Severinghaus, Thomas Blunier, Logan E. Mitchell, James E. Lee, Jon S. Edwards and Vasileios Gkinis, Nature Geoscience 7, 459–463 (2014) doi:10.1038/ngeo2147
For important articles Nature provides a review in the same issue that helps set a broader context for the primary article. In this case the review article was contributed by Eric W. Wolff, a respected researcher in paleoclimatology.
Palaeoclimate: Climate in phase, Eric W. Wolff, Nature Geoscience 7, 397–398 (2014) doi:10.1038/ngeo2165