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Observational records and climate projections provide abundant evidence that freshwater resources are vulnerable and have the potential to be strongly impacted by climate change, with wide-ranging consequences for human societies and ecosystems. Observed warming over several decades has been linked to changes in the large-scale hydrological cycle such as: increasing atmospheric water vapour content; changing precipitation patterns, intensity and extremes; reduced snow cover and widespread melting of ice; and changes in soil moisture and runoff. Precipitation changes show substantial spatial and inter-decadal variability. Over the 20th century, precipitation has mostly increased over land in high northern latitudes, while decreases have dominated from 10°S to 30°N since the 1970s. The frequency of heavy precipitation events (or proportion of total rainfall from heavy falls) has increased over most areas (likely). Globally, the area of land classified as very dry has more than doubled since the 1970s (likely). There have been significant decreases in water storage in mountain glaciers and Northern Hemisphere snow cover. Shifts in the amplitude and timing of runoff in glacier- and snowmelt-fed rivers, and in ice-related phenomena in rivers and lakes, have been observed (high confidence).

BRENDAN MAC KEY and SONG LI discuss two vital ingredients often overlooked in efforts to find solutions to global warming. Our generation must begin to care sufficiently about future generations, people in other countries, and the greater community of life, and demand our governments show international leadership in negotiating a new legally binding, equitable international climate agreement. Without this agreement, based on a world ethic of universal responsibility, our efforts will fail to solve global warming. Governments are wavering when leadership is needed. Everyone needs to take a stand, become a leader.

Mangrove systems occur extensively on low gradient tropical shorelines, where sedimentation enables resilience during sea-level rise (SLR). Within mangroves, inundation frequencies across the intertidal slope cause zonation of different species with elevation. This tight sea-level control of the seaward margin and zones within mangroves has been demonstrated by precise EDM survey. Hence species zones in mangroves are definitive indicators of sea-level position, and pollen distributions record the locations of different zones in the sedimentary record. Pollen stratigraphic records can be used to reconstruct Holocene sea-levels and show mangrove response to change. Mangrove response to sea-level rise has been investigated in Bermuda, the Cayman Islands, Tonga and southern New Guinea. Radiocarbon dating of stratigraphy determined a sediment accretion rate of 1 mm a1for the low island locations, and up to 1.5 mm a"1 in two estuaries of southern New Guinea. The IPCC SLR projections of 9-88 cm by 2100 equate to a rate of 0.9-8.8 mm a"1. Mangrove recession events and replacement by lagoon environments are shown to occur during more rapid sea-level rise. In Bermuda rates of SLR exceed 2 mm a1and the largest mangrove area having existed for the last 2000 years lost 26% area in retreat of its seaward edge during the last century. In Tonga, a large mangrove swamp persisted 7000-5500 yr BP during SLR of 1.2 mm a1, then retreated when the rate increased. In Cayman 20 km of mangroves died back between 4080 and 3230 yr BP, during SLR of 2.8-3.3 mm a1, to become a lagoon. In extensive swamps of southern New Guinea gradual Late Holocene retreat of mangrove zones occurred with SLR of 0.67 mm a1. Hence while low island mangroves are likely to be the most sensitive to projected SLR, continental margin mangroves will also suffer disruption and retreat.