  The observed global mean surface temperature increase from 1998 to 2012 was
slower than that since 1951. The relative contributions of all relevant factors
including climate forcers, however, have not been comprehensively analyzed.
Using a reduced-complexity climate model and an observationally constrained
statistical model, we find that La Nina cooling and a descending solar cycle
contributed approximately 50% and 26% of the total warming slowdown during
1998-2012 compared to 1951-2012. Furthermore, reduced ozone-depleting
substances and methane accounted for roughly a quarter of the total warming
slowdown, which can be explained by changes in atmospheric concentrations. We
identify that human factors played an important role in slowing global warming
during 1998-2012, shedding light on the evidence for controlling global warming
by reducing greenhouse gas emissions.
