RESUMO
We projected changes in weather extremes, hydrological impacts and vulnerability to food insecurity at global warming of 1.5°C and 2°C relative to pre-industrial, using a new global atmospheric general circulation model HadGEM3A-GA3.0 driven by patterns of sea-surface temperatures and sea ice from selected members of the 5th Coupled Model Intercomparison Project (CMIP5) ensemble, forced with the RCP8.5 concentration scenario. To provide more detailed representations of climate processes and impacts, the spatial resolution was N216 (approx. 60 km grid length in mid-latitudes), a higher resolution than the CMIP5 models. We used a set of impacts-relevant indices and a global land surface model to examine the projected changes in weather extremes and their implications for freshwater availability and vulnerability to food insecurity. Uncertainties in regional climate responses are assessed, examining ranges of outcomes in impacts to inform risk assessments. Despite some degree of inconsistency between components of the study due to the need to correct for systematic biases in some aspects, the outcomes from different ensemble members could be compared for several different indicators. The projections for weather extremes indices and biophysical impacts quantities support expectations that the magnitude of change is generally larger for 2°C global warming than 1.5°C. Hot extremes become even hotter, with increases being more intense than seen in CMIP5 projections. Precipitation-related extremes show more geographical variation with some increases and some decreases in both heavy precipitation and drought. There are substantial regional uncertainties in hydrological impacts at local scales due to different climate models producing different outcomes. Nevertheless, hydrological impacts generally point towards wetter conditions on average, with increased mean river flows, longer heavy rainfall events, particularly in South and East Asia with the most extreme projections suggesting more than a doubling of flows in the Ganges at 2°C global warming. Some areas are projected to experience shorter meteorological drought events and less severe low flows, although longer droughts and/or decreases in low flows are projected in many other areas, particularly southern Africa and South America. Flows in the Amazon are projected to decline by up to 25%. Increases in either heavy rainfall or drought events imply increased vulnerability to food insecurity, but if global warming is limited to 1.5°C, this vulnerability is projected to remain smaller than at 2°C global warming in approximately 76% of developing countries. At 2°C, four countries are projected to reach unprecedented levels of vulnerability to food insecurity.This article is part of the theme issue 'The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels'.
RESUMO
Understanding how 1.5 °C pathways could adjust in light of new adverse information, such as a reduced 1.5 °C carbon budget, or slower-than-expected low-carbon technology deployment, is critical for planning resilient pathways. We use an integrated assessment model to explore potential pathway adjustments starting in 2025 and 2030, following the arrival of new information. The 1.5 °C target remains achievable in the model, in light of some adverse information, provided a broad portfolio of technologies and measures is still available. If multiple pieces of adverse information arrive simultaneously, average annual emissions reductions near 3 GtCO2/yr for the first five years following the pathway adjustment, compared to 2 GtCO2/yr in 2020 when the Covid-19 pandemic began. Moreover, in these scenarios of multiple simultaneous adverse information, by 2050 mitigation costs are 4-5 times as high as a no adverse information scenario, highlighting the criticality of developing a wide range of mitigation options, including energy demand reduction options.
RESUMO
The atmospheric histories of two potent greenhouse gases, tetrafluoromethane (CF4) and hexafluoroethane (C2F6), have been reconstructed for the 20th century based on firn air measurements from both hemispheres. The reconstructed atmospheric trends show that the mixing ratios of both CF4 and C2F6 have increased during the 20th century by factors of approximately 2 and approximately 10, respectively. Initially, the increasing mixing ratios coincided with the rise in primary aluminum production. However, a slower atmospheric growth rate for CF4 appears to be evident during the 1990s, which supports recent aluminum industry reports of reduced CF4 emissions. This work illustrates the changing relationship between CF4 and C2F6 that is likely to be largely the result of both reduced emissions from the aluminum industry and faster growing emissions of C2F6 from the semiconductor industry. Measurements of C2F6 in the older firn air indicate a natural background mixing ratio of <0.3 parts per trillion (ppt), demonstrating that natural sources of this gas are negligible. However, CF4 was deduced to have a preindustrial mixing ratio of 34 -1 ppt (-50% of contemporary levels). This is in good agreement with the previous work of Harnisch et al. (18) and provides independent confirmation of their results. As a result of the large global warming potentials of CF4 and C2F6, these results have important implications for radiative forcing calculations. The radiative forcings of CF4 and C2F6 are shown to have increased over the past 50 years to values in 2001 of 4.1 x 10(-3) Wm(-2) and 7.5 x 10(-4) Wm(-2), respectively, relative to preindustrial concentrations. These forcings are small compared to present day forcings due to the major greenhouse gases but, if the current trends continue, they will continue to increase since both gases have essentially infinite lifetimes. There is, therefore, a large incentive to reduce perfluorocarbon emissions such that through the implementation of the Kyoto Protocol, the atmospheric growth rates may decline in the future.