A research team led by Tom Marchitto has uncovered evidence supporting an important role for the Sun in regional-scale climate variability. They found that slow variations in solar output have nudged the circulation of the tropical Pacific Ocean toward states resembling El Niño and its opposite phase, La Niña. El Niño and La Niña are immensely important for year-to-year climate variability, especially when it comes to precipitation: for example, they explain why southern California may see floods and mudslides one year, and droughts and wildfires the next year. While different computer models agree on many aspects of future climate change, there is less agreement about the future of El Niño. Marchitto and colleagues hope that an improved understanding of El Niño’s response to solar variability may lead to better projections of its response to the warming caused by greenhouse gases.
The team used an ocean sediment core from off the coast of Baja California to reconstruct sea surface temperatures over the past 14,000 years. They then compared that temperature history to estimates of changes in the Sun’s brightness through time. They found that over centuries, when the Sun is a little brighter, ocean temperatures were, counter-intuitively, slightly colder. This cooling is what happens during La Niña. That is, a brighter Sun appears to result in a tendency for more La Niña-like conditions. This response, which appears in some computer models, is called a “thermostat” because the tendency for the Earth’s surface to warm when the Sun is brighter is counteracted by the cooling of the tropical Pacific Ocean.
Changes in the Sun’s brightness over recent decades have been exceedingly small. This variability pales in comparison to the influence of greenhouse gases, such that the bulk of recent global warming cannot be ascribed to the Sun. The new results imply that if the solar thermostat mechanism were happening now, then even more of the current warming would need to be attributed to greenhouse gases. In contrast to a brighter Sun, greenhouse warming itself is expected to cause the trade winds to slacken, which would tend to push the system back toward the El Niño (warm) state.
Other members of the team are Raimund Muscheler (Lund University, Sweden), Joseph Ortiz (Kent State University), Jose Carriquiry (Universidad Autónoma de Baja California, Mexico), and Alexander van Geen (Columbia University). The team’s study was published in the December 3 issue of Science.