TIMESCALES FOR DETECTING SIGNIFICANT ACCELERATION IN SEA-LEVEL RISE

There is observational evidence that global mean sea levels (MSL) are rising and there is considerable concern that the rate of rise will accelerate throughout the 21^st century, significantly threatening growing coastal communities. Therefore, early detection of a significant increase in the rate of MSL rise is crucial to enable adequate adaption, particularly if it is at the postulated higher rates of rise. However, while we: (i) have high confidence from paleo sea level data and from a few long tide gauge records that there was an increase in the rate of MSL rise during the late 19^th and early 20^th centuries; and (ii) are virtually certain from tide gauge and radar altimetry observations that global MSL has risen over the 20^th century at average rates of 1.7mm/yr and at rates almost double that since 1993; there appears to be uncertainty regarding two related questions which have caused some to doubt the comparatively large rises in MSL projected for the 21^st century. First, is there evidence for significant acceleration in MSL since early in the 20^th century or are observed changes since then essentially linear or even negative? Second, are the recent high rates of rise measured by radar altimetry since 1993, significantly larger than rates observed at other times in the past two centuries, or are they just associated with decadal variability? In this presentation we will examine both questions, which in part relate respectively to two methods that have tended to be used to detect accelerations, namely: (i) fitting quadratic equations to tide gauge records, with the acceleration defined as twice the quadratic coefficient; (ii) estimating linear trends for consecutive, overlapping time periods of different lengths. We will expand on each question by briefly reviewing the literature and then we will provide fresh insight by applying these two methods, to synthesized datasets created by combining tide gauge records or global reconstructions up to present with different projections of MSL ranging from 0.5-2m from the present to 2100. We find that using these methods the earliest we may be able to identify that MSL are rising at a rate statistically significantly higher than at any other time over the two past two centuries is the mid to late 2020’s for a MSL rise of between 0.5-0.8m by 2100, and early 2020’s for larger rise of 1-1.5m.