With ocean temperatures rising worldwide, scientists are looking to past instances of global warming to better understand future potential effects. In a new study, Minshull et al. investigate methane release from the seafloor during a period of global warming that occurred around 56 million years ago. The study suggests this methane release was slower and more modest than some researchers have hypothesized.
The team focused on the Paleocene-Eocene Thermal Maximum (PETM), a time when proxy records indicate that Earth’s sea surface temperatures increased by up to 9°C and reached their highest point so far in the past 65 million years. This hot period lasted for about 100,000 years and caused extinction of many species.
Climate records of the past, built from analysis of fossils in sediment layers, show that close to the start of the PETM large amounts of carbon rich in the carbon-12 (12C) isotope entered the oceans and atmosphere. Scientists have hypothesized that a massive release of methane from the seafloor was responsible for this carbon input. Warm seawater could have “melted” icelike methane hydrate buried beneath the seafloor, the theory goes, releasing the greenhouse gas into the ocean and potentially warming the planet even further.
The researchers used a mathematical model to investigate how this methane release event might have unfolded. They simulated the effects of PETM ocean warming on sediments that may have contained methane hydrate during the Paleocene and tracked how methane transport mechanisms would have affected its release into seawater.
The researchers found that although the PETM could have caused rapid methane hydrate breakdown in deep-sea sediments, methane release from the sediments into the ocean would have taken hundreds to thousands of years. In the simulations, most of the methane remained trapped in sediment pores.
In order for methane hydrate breakdown to cause the 12C increase of the early PETM, the results suggest, much more hydrate must have existed worldwide than expected given the warmth of the late Paleocene ocean. The authors say their findings challenge the hypothesized role of methane hydrates in the PETM. They also raise important questions about the potential for breakdown of present-day methane hydrates to exacerbate future climate change, although current warming rates are much higher than those during the PETM. (Geophysical Research Letters, doi:10.1002/2016GL069676, 2016)
—Sarah Stanley, Freelance Writer