The National Oceanic and Atmospheric Administration's latest Annual Greenhouse Gas Index, released Friday, found GHG emissions continued to increase in 2013, contributing to a 1.5% increase between 2012 and 2013.
NOAA says the increase means the combined heating effect of human-emitted, long-lived greenhouse gases currently in the atmosphere has increased by 1.5% in one year, and 34% since 1990.
"We continue to turn the dial up on this 'electric blanket' of ours without knowing what the resulting temperatures will be," said James Butler, Ph.D., director of the Global Monitoring Division of NOAA's Boulder-based Earth System Research Laboratory. "We know that the world is getting warmer on average because of our continued emissions of heat-trapping gases."
Scientists at NOAA calculate the AGGI each year from several decades of atmospheric data collected through an international cooperative air-sampling network of up to 80 sites around the world.
2013 AGGI results
In 2013, carbon dioxide concentrations for the first time in recorded history exceeded 400 parts per million(ppm) at Mauna Loa —considered a "global benchmark" monitoring site—in early May. This year, CO2 exceeded 400 ppm at Mauna Loa in mid-March, two months earlier than last year.
Concentrations at Mauna Loa have continued to top 400 ppm throughout much of April and are expected to stay at historic high levels through May and early June, NOAA expects, dropping in early summer only as trees and plants in the Northern Hemisphere begin to take up CO2 during the growing season.
Related: Obama Administration Calls on Ag to Cut Methane Emissions
Carbon dioxide was responsible for 87% of the increase this past year, NOAA said. The annual rise in CO2 is consistent with trends in fossil fuel emissions and carbon dioxide uptake by the ocean and land ecosystems. Historically, about half of the carbon dioxide emitted is removed from the atmosphere by the ocean and land vegetation.
Atmospheric methane concentrations stabilized during the late 1990s and early 2000s, but have been rising again since 2007. The largest sources of methane emissions are naturally occurring and come from wetlands in the tropics.
Human-caused methane emissions, NOAA says, come from the production of coal, oil and natural gas, livestock, rice agriculture and waste. Nitrous oxide continues to rise at a steady rate. Nitrous oxide occurs naturally in the atmosphere, but has been increasing in recent decades owing largely to human-driven emissions.
The report follows an Obama Administration pledge earlier this year to reduce methane emissions, in part from agriculture.
During his address announcing the methane plan, President Obama said the efforts will include collaboration between the Environmental Protection Agency, the Department of Energy and the USDA to work with dairies to install methane digesters and implement other emissions mitigation tactics.
Sens. Mike Johanns, R-Neb., and Deb Fischer, R-Neb., have since pushed back on the plan, suggesting it will cost farmers as much as $27,000 to implement fully.