Climate Change: the Science Basis. Models and Scenarios for Climate and Energy future development

According to the Planet “vital signs”, climate is changing mainly due to anthropogenic factors. This work aims to address the main aspects of climate change with a strict approach, starting from its mechanisms and impacts on life, switching to mitigation and adaptation actions, going through the most updated scientific literature. Seven climate development scenarios are worked out using Java Climate Model (JCM). Global Mean Temperature (GMT) started to increase in 1880 and is getting momentum: 2016, 2015, 2017, 2018, 2014 are the “top five” warmest years on record. 2019 is on track to be the new third. The anomaly w.r.t. 1880 ranges from 0.74 °C to 0.94 °C. Arctic Sea Ice Minimum (ASIM) lowest was 3.8-3.9 Mkm2 in 2012, 4 Mkm2 lower than the ‘80s values. ASIM “top five” smallest occurred between 2015 and 2019 and its average extent diminishing rate is 12.8% per year. Antarctica and Greenland keep losing mass since 2002. W.r.t. then Antarctica lost 1,870 (± 175) Gt, while Greenland lost 3,771 (± 98) Gt, dropping now by 127 (± 39) and 286 (± 21) Gt per year respectively. The cumulative glacier mass balance - negative 63 of the last 68 years - shows a 20 m water equivalent (w.e.) loss from 1980 and is currently falling by 847-1,036 mm w.e. per year. The 93% (17x10^22 J) of the excess heat produced since 1970 has been absorbed by oceans, increasing their top 700 m temperature by 0.09-0.13 °C per decade. Global Marine Sea Level (GMSL) increased by 235 (± 5) mm w.r.t. 1880 as a result. Never has the atmospheric CO2 concentration exceeded 300 ppm during the last 1 Myear, but it started to climb from 311 ppm in 1950 to the today’s 414.83 ppm. These Planet’s alterations are producing several impacts on natural systems. Replacing fossil fuel demand with a renewable-based one, together with energy efficiency improvement and demand reduction are the most effective impact mitigation strategies. Nevertheless, a certain grade of modification on Planet’s life due to climate change is likely, so crucial are adaptation actions too, including smart planning of cities, preservation of ecosystems and literacy work about the perception of climate change. Projections on future climate are made using climate models. Through econometric, demographic and energy assumptions, they provide estimates then checked against real observations and gathered in scenarios, whose outputs mainly show strong correlation towards a further warming in the years to come. Among the scenarios provided, three “normative” Stabilization ones (ST1.5 ST2.0 and ST3.0) aim to stabilize the GMT increase to 1.5, 2.0 and 3.0 °C by 2100. Three “predictive” scenarios focus on the energy system evolution, with the first two Current Policies accounting for a fossil-based one (CP1) and a more balanced one (CP2), and the New Policies Scenario (NPS) including climate policies announced after COP 21. The last “exploratory” Sustainable Development Scenario (SDS) models the most preferable future possible, with social equity, fast energy transition, cleaner air and water. Looking at the results, the only two scenarios not exceeding 2.0 °C are ST1.5 and SDS. The sole SDS shows a peak-and-decline GMT trend, meaning that according to the others the GMT rise will continue after 2100. Given the current socioeconomic, energy and demographic conditions seems extremely unlikely not to exceed the 1.5-2.0 °C increase in GMT by 2100. Science has never been clearer than now. Climate change is no more ignorable.