Washington: A new research has suggested that a natural shift to stronger warm El Nino events in the Pacific Ocean may be responsible for a substantial portion of the global warming recorded during the past 50 years.
Lead author Dr. Roy Spencer , a principal research scientist in UAH`s Earth System Science Center, said that their modelling shows that natural climate cycles explain at least part of the ocean warming they`ve seen since the 1950s.
Spencer and co-author Dr. Danny Braswell used all of the usual climate modeling forcings - including carbon dioxide and other greenhouse gas enrichment - in their study, but also plugged the observed history of El Nino ocean warming and La Nina ocean cooling events into their model to calculate the 61-year change in global ocean temperature averages from the sea surface to a depth of 2,000 meters.
When they ran their ocean model without ENSO, they arrived at the same general conclusions as the more complex general circulation climate models.
When they added data from past El Nino and La Nina events as only a change in ocean mixing, the model indicated a climate system that is slightly less sensitive to CO2-induced warming than has been believed.
But the biggest change was when the model was allowed to change cloud cover with El Nino and La Nina in the same way as has been observed from satellites.
The results suggest that these natural climate cycles change the total amount of energy received from the sun, providing a natural warming and cooling mechanism of the surface and the deep ocean on multi-decadal time scales.
Spencer said that as a result, because as much as 50 percent of the warming since the 1970s could be attributed to stronger El Nino activity, it suggests that the climate system is only about half as sensitive to increasing CO2 as previously believed.
The study has been published in the Asia-Pacific Journal of Atmospheric Science.