Do AR4 ao AR5 - o caminho percorrido
Pedro Leite da Silva Dias (*)
Laboratório Nacional de Computação Científica/MCTI
Instituto de Astronomia, Geofísica e Ciêncas Atmosféricas/USP
Seminário:
MUDANÇAS CLIMÁTICAS: IMPACTOS, VULNERABILIDADE E ADAPTAÇÃO
O sistema terrestre: acoplamento entre a física, biogeoquímica e a forçante humana
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•Modelos no AR5 têm complexidade inédita: •Efeito do derretimento do permafrost
•Fertilização da biota pelo CO2 •Disponibilidade de nutrientes
•Ciclo biogeoquímico de gases traço •Aerossois e nuvens
•Interação das correntes oceânicas com o gelo… •Cenários econômicos interativos…
The Arctic Ocean is now projected to be ice-free during the summer by mid-century under a high emissions scenario, instead of the end of century as in previous reports.
Highlights of the AR5
The conclusion that much of the warming over the past 50 years is due to human activities is now “extremely likely,” upgraded from “very likely” in the last report.
Estimates of future sea level rise have been significantly increased due to a better understanding of the movement of ice sheets in a warming climate.
Each IPCC report has been progressively stronger in attributing climate change to human activities.
• The AR5 contains the strongest statement yet, saying it is “extremely likely” (a greater than 95 percent chance) that human activities are “the dominant cause of the observed warming” since the 1950s.
• The Third Assessment (2001) made a similar statement with
approximately 66 percent certainty, while the Fourth
Assessment Report (AR4) (2007) found that “most of the observed increase in global average temperatures since the
mid-20th century is very likely (greater than 90 percent chance) due to the observed increase in anthropogenic greenhouse gas concentrations.”
The AR4 concluded that “warming of the climate system is unequivocal.”
• The AR5 goes further, concluding that many observed
changes (warming of the atmosphere and ocean, sea level rise and melting ice) are “unprecedented over decades to millennia.”
• New atmospheric temperature measurements in the AR5 show an estimated warming of 0.85 degrees Celsius since 1880 with the fastest rate of warming in the Arctic. The AR4 estimated the average warming across the globe over the past century (1906-2005) was 0.74 C.
The AR5 report has significantly increased projected sea level rise over the next century, due to new research that improves understanding of ice sheet movement and melting.
• The new projections show an increase of 0.26-0.55 meters) by 2100 under a low emissions scenario and 0.52-0.98 meters under the high emissions scenario.
• The AR4 did not include some of the effects of ice sheet movement due to warming, and therefore published much lower estimates in the range of 0.18-0.38 meters under a low emissions scenario and 0.26-0.59 meters) under a high emissions scenario for sea level rise by 2100.
The AR5 projects it is likely (greater than 66 percent chance) that the Arctic Ocean will be ice-free during part of the summer before 2050 under a high emissions scenario.
• This represents a large shift from the AR4, which estimated that the Arctic Ocean would not be ice-free during the summer until late in the 21st century. The AR5 finds that Arctic sea ice surface extent has decreased by 3.5-4.1
percent per decade (9.4-13.6 percent during summer), which is higher than the AR4 estimate of 2.1-3.3 percent per decade (5-9.8 percent during summer). The AR5 finds these changes unprecedented in at least the last 1450 years.
• The AR5 also states that scientists have “high confidence” (80 percent chance) that glaciers have shrunk worldwide, and that the Greenland and Antarctic Ice Sheets have lost mass over the past two decades. The report notes with “very high confidence” (90 percent chance) that ice loss from Greenland has
accelerated during the past two decades. Greenland is now losing about 215 gigatonnes (Gt) per year of ice, while the rest of the world’s glaciers lose about 226 Gt per year.
After a period of rapid warming during the 1990s, global mean surface temperatures have not warmed as rapidly over the past decade.
• The AR5 notes there are “differences between simulated and observed trends over periods as short as 10-15 years (e.g., 1998-2012)”. It concludes that the
recent reduction in surface warming is probably due to a redistribution of heat in the ocean, volcanic eruptions, and the recent minimum in the 11-year solar cycle.
• Most importantly, the report specifically points out that these trends should not undermine our confidence in the “big picture” of our understanding of climate change: “trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.”
• In addition, there is new research proposing explanations for the recent trends that did not make the deadline to be included in the AR5. One paper suggests
that some of this “lost” heat is actually in the deep ocean, while another notes
that the warming “pause” is explained by the unusual number of La Niña events in the Pacific Ocean. The second paper by Yu Kosaka and Shang-Ping Xie states that the “current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue.”
The AR5 relates different carbon “budgets” – an accumulated amount of carbon emissions over time — to the chances of
average warming exceeding 2 degrees above 1861-1880 levels. • Governments have set an international goal of limiting average
warming to 2 C. For the world to have a 50 percent chance of staying below 2 C of warming by 2100, the AR5 identifies a greenhouse gas emissions budget of 840Gt of carbon.
• More than half of that (over 531GtC) has already been emitted. At current emission rates (around 10 GtC per year), we will use up our carbon budget in just 30 years.
The report describes several alternative scenarios of 21st century greenhouse gas concentrations and global temperatures, each associated with different cumulative carbon budgets.
• Three scenarios represent potential pathways with less warming under various forms
of mitigation policy.
• The fourth represents more of a business-as-usual case, with emissions in the 21st century three to four times larger than the emissions before the 20th century and the highest level of warming in any scenario.
• For comparison, here are the IPCC’s projections in four key areas: from the 2013 AR5, in bold – from the 2007 AR4, in regular type:
• Probable temperature rise by 2100: 1.5-4°C under most
scenarios – from 1.8-4°C
• Sea level rise: very likely faster than between 1971 and
2010 – by 28-43 cm
• Arctic summer sea ice disappears: very likely it will
continue to shrink and thin – in second half of century
• Increase in heat waves: very likely to occur more
frequently and last longer – increase very likely
Summary - main conclusions
Irreversibility and Abrupt Change
Cruz et al. 2009
Análise de espeleotemas - cavernas -
Global means and average of all CMIP5 models show indicate a broad picture
Percent change in precipitation from the years 1980-1999 to 2080-2099 under the A1B scenario. Brown indicates a reduction in
precipitation and green an increase. The per cent change in the precipitation averaged over all models is shown in the lower right hand corner.
However,
regionally, there are significant differences…
Um exemplo de incerteza nos
modelos que nos afeta diretamente:
•Aquecimento global e floresta amazônica:
•Exemplo de potencial efeito
“catastrófico” frequentemente usado:
Haddley Center
Retrocesso da floresta amazônica por
mudança climática
1850 2000 2100
Exemplo de extremo nos modelos do
IPCC: Haddley Center
Entretanto, há considerável discrepância entre
modelos semelhantes…(C
4MIP-IPCC -2007)
•Climate change will very likely affect most forests in Amazonia during the course of the 21st century, but the direction and intensity of the change are uncertain, in part because of
differences in rainfall projections. In order to constrain this uncertainty, we estimate the probability for biomass change in Amazonia on the basis of rainfall projections that are weighted by climate model performance for current conditions.
• We estimate the risk of forest dieback by using weighted rainfall projections from 24 general
circulation models (GCMs) to create probability density functions (PDFs) for future forest biomass changes simulated by a dynamic vegetation model (LPJmL).
•
.
• The uncertainty associated with the long-term effect of CO2 is much larger than that
associated with precipitation change. This underlines the importance of reducing uncertainties in the direct effects of CO2 on tropical ecosystems.
•Our probabilistic assessment of biomass change suggests a likely shift towards increasing biomass compared with nonweighted results. Biomass estimates range between a gain of 6.2 and a loss of 2.7 kg carbon m)2 for the Amazon region, depending on the strength of CO2 fertilization
Moss et al (2010) The next generation on scenarios for climate change research and assessment. Nature 463:747-756
Mensagem:
•não devemos analisar somente os modelos
que indicam extremos….
•É preciso ponderar os modelos em função da
Aumento do conhecimento e entendimento sobre o sistema climático leva a diminuição da incerteza?
Consenso:
• a incerteza nos cenários climáticos no próximo levantamento do IPCC (AR5) deverá ser maior que nos levantamentos anteriores!!!!
• Enquanto nosso conhecimento sobre certos fatores
aumenta, aumenta também nosso entendimento sobre o efeito de fatores que não considerávamos ou que sequer reconheciamos !!!
Mudanças Climáticas: certezas, incertezas e desafios.
Papel do Cientista
na Formulação de
Políticas Públicas
1. Cientista Puro – afastado dos formuladores de políticas públicas -
3. Cientista Defensor de Causas – focado numa agenda política (econômica, religiosa etc...) em particular.
2. Cientista Árbitro – focado em questões que podem ser resolvidas pela ciência
4. O “Corretor Honesto” de alternativas políticas - este lida diretamente com as incertezas científicas e interage com o formulador de políticas públicas
Baseado em (R. Pielke Jr - The Honest Broker - 2007 )
