Increase in the rate of ocean warming

Increased human-emitted greenhouse gases are the major cause of the current energy imbalance for the Earth, and subsequently to the multi-decadal observations of ocean temperature increase. Quantifying the rate of ocean temperature (heat content) changes is challenging because of the necessary duration of measurements, issues of instrument accuracy, and challenges of adequate ocean coverage. In addition, statistical analysis of the global-temperature measurements is subject to some uncertainty. Traditionally, linear trendlines have been used to quantify long-term warming rates. However, linear trends are unable to capture changes to warming rates (acceleration or deceleration).

Here, results are presented that match up-to-date ocean temperature measurements with alternative trendline analyses. In particular, two other (non-linear) approaches can provide useful information. Based on the new methods, we show an increase in ocean warming for the upper 2000 meters since ~ 1958. The energy imbalance estimates reveal a near energy balance in 1958-1973 that changes to a warming rate of 0.57-0.70 W m^-2 during 2003-2018. The best estimate of ocean heat content for the time period from 1971 to 2010 is 0.36 - 0.41 W m^-2. We are also able to show the penetration of ocean warming from the upper layers (upper 300 m) to the deeper regions (down to 2000m).

The methodologies employed here are termed "local" methods that allow a number of degrees of freedom to be able to impose a time scale that treats high frequency behavior as noise. Other degrees of freedom help accommodate higher order (non-linear) behaviors in the trend. The two employed methods are piece linear fit (PLF) and a locally weight scatterplot smoothing (LOWESS). For both approaches, span widths from 2-30 years were used and optimal spans were found to be in the 10-20 year range.

The methods were applied to three ocean warming datasets. All three datasets show highly consistent warming since 1958 and a significant acceleration in the warming rate. The resulting mean ocean warming for the upper 2000m accelerated from -0.01~0.17 Wm^-2 (1958-1973), to 0.13~0.27 Wm^-2 (1973-1988), to 0.43~0.58 Wm^-2 (1988-2003), to 0.57~0.71 Wm^-2 (2003-2018).