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Earth will hit 1.5°C around 2030, human influence on climate "unequivocal": UN climate report

By Anuj Cariappa
Google Oneindia News

New Delhi, Aug 09: From record-breaking temperatures, raging wildfires to devastating floods the world has seen it all in the recent years. The IPCC report on climate change released today, set out the stark reality of global warming.

    UN's IPCC report on climate change suggests 'code red for humanity' | Global warming | Oneindia News
    UN unveils landmark climate change report: Key highlights

    The report, Climate Change 2021: the Physical Science Basis reviewed by hundreds of experts, scientists including India, has emerged as one of the most significant documents informing world leaders on the state of climate ahead of the UN climate negotiations (COP26) in November.

    In its 40-page report, the UN climate panel details about changing climatic conditions, whether climate crisis caused due to human influence; future scenarios and how soon we are likely to hit the 1.5 degrees C and 2 degrees C thresholds.

    The report assumes significance for India because IPCC will, for the first time, provide a localised outlook, with maps and atlas.

    India has seen severe impacts of the climate change, right from devastating floods, a spate of cloudbursts in Uttarakhand and Himachal Pradesh, and deadly lightening incidents.

    UN Climate Report 2021: Key Highlights

    • Scientists have no doubt that human activities have warmed the planet. Rapid and widespread changes have occured in the planet's climate and some impacts are now locked in.
    • Improved attribution science finds evidence of humankind's impact throughout the climate system, human-caused emissions are now responsible for an altered, less stable planet.
    • The planet will warm by 1.5°C in all scenarios. In the most ambitious emissions pathway, we reach 1.5°C in the 2030s, overshoot to 1.6°C, with temperatures dropping back down to 1.4°C at the end of the century.
    • Scientists are clear on the need to tackle greenhouse gases other than CO2 in the near-term, emissions of methane - a powerful greenhouse gas - are of particular concern.
    • The natural world will be damaged by further warming, and so the land and ocean ecosystems have a limited capacity to help us solve the climate challenge.
    • Decision makers need to implement net zero plans if we are to stop warming. Carbon dioxide removal is a crucial net zero tool, but one that will only be useful when accompanied by swift and deep emissions reductions.
    • Estimates of the remaining carbon budget-a simplified way of assessing how much more CO2 can be released-have been improved since previous reports, but the carbon budget remains broadly unchanged.

    Human influence on the climate

    • Scientists say it is unequivocal that climate change is caused by us. Human influence has warmed the climate system, and widespread and rapid changes in climate have occurred.
    • Human influence has warmed the climate at a rate that is unprecedented in at least the last 2000 years.
    • These statements build on the certainty in previous IPCC assessments, this SPM states that increases in GHG concentrations since around 1750 are unequivocally caused by human activities.
    • In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years and concentrations of methane and nitrous oxide, both significant GHGs, were higher than at any time in at least 800,000 years.
    • The rate of warming is speeding up: global surface temperatures increased faster since 1970 than in any other 50-year period over at least the last 2000 years.
    • Human-caused emissions are responsible for almost the entirety of global warming.

    How have we altered the planet?

    • Many consequences of ongoing climate change are irreversible on centennial to millennial time scales, especially for changes in the ocean, ice sheets and global sea levels.
    • The scale of recent changes experienced across the climate system and much of its present state are unprecedented over thousands of years.
    • The last decade saw Arctic sea ice at its lowest level since 1850.
    • Tipping points are included in the report because, despite having a lower chance of happening, they could have devastating impacts. Low-likelihood outcomes, such as ice sheet collapse, abrupt ocean circulation changes, some compound extreme events and warming substantially larger than the assessed very likely range of future warming cannot be ruled out and are part of risk assessment.
    • The further we go past 1.5°C, the more we build unpredictable and serious risk into our world. These tipping points could occur at global and regional scales, even for global warming within the very likely range of the considered emissions scenarios. Abrupt responses and tipping points of the climate system, such as strongly increased Antarctic ice sheet melt and forest dieback, cannot be ruled out.
    • Global mean sea level rise has risen faster since 1900 than any other time over the last 3000 years.
    • Marine heatwaves have doubled in frequency since the 1980s, human influence has very likely contributed to most of them since at least 2006.
    • Mountain and polar glaciers are committed to continue melting for decades or centuries, while the loss of permafrost carbon by thawing is irreversible when considered over a thousand year period.
    • Global mean sea level rise above the likely range - by up to 2 m in 2100 and 5 m by 2150 - cannot be ruled out in the highest emissions scenario, due to deep uncertainty in ice sheet processes.
    • Sea level rise will continue for hundreds to thousands of years even in the most ambitious climate pathways.

    Changing climate systems - increasing fires and floods

    • Since AR5 there have been major updates on the evidence and knowledge of extreme weather events driven by climate change. New developments in attribution science-in which scientists analyse how much human activities have influenced particular weather events-has made it clear that we are contributing to the increase in both the likelihood and severity of extreme heat, precipitation, droughts and tropical cyclones.
    • Most of the planet is already weathering hot extremes (including heatwaves), including North America, Europe, Australia, big chunks of Latin America, west and east Southern Africa, Siberia, Russia, and across Asia (Figure SPM 3, page 14). Some of the recent hot extremes would have been extremely unlikely to occur without human influence.
    • Less is known about drought, but there is enough evidence to show that North Eastern South Africa, the Mediterranean, Southern Australia, the West Coast of North America in particular are dealing with increased drought.
    • Northern Europe and parts of North America and Southern Africa are dealing with heavier precipitation, but more data is needed to tell the story elsewhere.
    • It is likely that the global proportion of Category 3-5 tropical cyclones have increased over the last 40 years. There is high confidence that human-induced climate change means tropical cyclones bring heavier, more intense precipitation.
    • Every bit of warming matters: projected changes in extremes are larger in frequency and intensity with every additional increment of global warming.. Fires and floods of the type we have seen this summer are now supercharging as human-induced warming changes the climate system.
      o Hotter, drier extremes: heat extremes that might have only occurred infrequently without human-induced warming, rapidly supercharge in intensity and frequency as temperatures rise.
    • Extreme rainfall events are also predicted to become more frequent and bring a significant and growing amount of water when they do hit.
    • A crucial update to AR6 WGI is the analysis of compound events - for example heatwaves and droughts happening close together or even at the same time. This poses a particular risk as they often leave communities with little or no time to recover between events. The report finds that human influence has likely increased the chance of compound extreme events since the 1950s.

    What does this say about Paris Agreement temperature targets?

    • The Paris Agreement sets the goal temperature rise by the end of the century to be no more than 2°C, and preferably no more than 1.5°C. The WGI report is clearer than ever: both the 1.5°C and 2°C warming limits will be exceeded during the 21st century unless we deeply reduce CO2, along with other greenhouse gas emissions, to net zero around or after 2050.
    • This is unprecedented territory, as the last time earth's surface temperature was above 2.5°C higher (compared to preindustrial levels) was over 3 million years ago.
    • WGI outlines what happens to future temperature under five socio-economic pathways (SSPs) that the world could follow.
    • In all but the lowest emissions scenario (SSP1-1.9), a global warming of 1.5°C will be exceeded in the near future (between 2021 and 2040) and will stay above 1.5°C till the end of the century.
    • The lowest emissions scenario sees us stay below 1.5°C, after a temporary overshoot of less than 0.1°C, before carbon is removed from the atmosphere and temperatures are brought back down.
    • As global average temperatures continue to rise, the risk of a temporary average of 1.5°C in at least one of the next five years increases. But the report stresses that the occurrence of a single year of temperature change above a certain level, like 1.5°C and 2°C, does not imply that the global warming level has been breached.

    How does this compare to the 2018 IPCC Special Report on 1.5 (SR1.5), which found that - at current rates - global warming would cross 1.5°C between 2030 and 2052?

    • The reports are not directly comparable, as this report looks at a broad set of scenarios and SR1.5 only considers linear trends (IPCC Q&A). If the methods of the two reports were reconciled, the SR1.5 estimate of when 1.5°C global warming is first exceeded would be close to the best estimate reported in AR6. As such, it does not mean that we are actually warming quicker than previously thought.

    What does the report say about our current emissions pathway?

    • SSP2-4.5 most closely mirrors the emissions trajectory that the total ambition of climate pledges (NDCs) currently has us on. CO2 continues to rise and stabilise around the mid century, before finally starting to decrease, with the steepest drop just before the end of the century.
    • Methane and sulphur dioxide continue to rise, starting to trend downwards around mid century.
    • Nitrous oxide most noticeably trends upwards and doesn't decrease until well into the second half of the century. Best estimate of 2.7°C at 2100.

    Future projections and implications for net zero plans

    • Capacity for land and ocean sinks to take up carbon is not infinite, and we see them carrying the biggest carbon load in the two most ambitious scenarios. This ability doesn't proportionately rise with GHG emissions. With the three other assessed scenarios, natural sinks sequester a rapidly shrinking share of the emissions.
    • In the most ambitious pathway (SSP1-1.9), natural sinks absorb 70% of emissions.
    • In the pathway our current policies and climate plans most closely align with (SSP2-4.5), only 54% of emissions are taken up by natural sinks, leaving the rest to be removed by carbon dioxide removal technologies.
    • We are currently dealing with our past carbon emissions, and we want to avoid adding more for future generations to deal with. From a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions.

    Note: Although not specifically mentioned in this report (as it is the job of the WGIII report released in March 2022), it is useful to refer back to the IPCC SR1.5 for pathways and trajectories. The headline being In model pathways with no or limited overshoot of 1.5°C, global net anthropogenic CO2 emissions decline by about 45% from 2010 levels by 2030 (40-60% interquartile range), reaching net zero around 2050 (2045-2055 interquartile range)

    • Carbon dioxide removal (CDR) is required to achieve net zero in all scenarios, as outlined previously in the IPCC 1.5 Special Report. It will influence vital life support systems such as water availability, food production, and biodiversity.
    • CDR covers a broad range of methods, from afforestation, wetland restoration to direct air carbon capture and storage (DACCS) and ocean fertilisation.

    Note: Policy decisions will determine whether this influence is positive or negative, but this will not be addressed here. AR6 WGIII will address this.

    • As CO2 emissions levels continue to rise, the ocean and land carbon sinks are projected to be less effective at slowing the accumulation of CO2 in the atmosphere.
    • The way in which the carbon cycle will react (called feedback, or feedback loops) becomes more uncertain, as well as more important, under a high emissions CO2 scenario. The further we push the climate system, the more unpredictable it will become, but those responses will be of a serious magnitude.
    • Carbon removal is not 'like-for-like'. When a tonne of CO2 is emitted into the atmosphere, the effect it has on atmospheric CO2 is greater than when a tonne of CO2 is removed by CDR method, because of the response of the land and ocean. The report states "the atmospheric CO2 decrease from anthropogenic CO2 removals could be up to 10% less than the atmospheric CO2 increase from an equal amount of CO2 emissions, depending on the total amount of CDR"
    • The range of climate sensitivity has narrowed since the last IPCC assessment cycle. The AR6 assessed best estimate is 3°C with a likely range of 2.5°C to 4°C, compared to 1.5°C to 4.5°C in AR5, which did not provide a best estimate

    Tackling greenhouses gases - it's more than CO2

    • This report finds that the amount of CO2 that can still be released is about 400 Gt of CO2, if we are to stand a 67% chance of staying below 1.5 (Compared to the 1850-1900 period).
    • This budget is of a similar magnitude that was assessed in SR1.5, and slightly larger than AR5, due to improvements in how scientists calculate the budget.
    • AR6 is the first IPCC assessment to include a chapter (Chapter 6) dedicated to so-called 'short-lived climate forcers', like aerosols, particulate matter and other reactive gases (like ozone) that exist in the atmosphere for a few hours to a couple of months (they also include methane, which has a lifetime of about 12 years).
    • The report finds that methane and nitrous oxide concentrations are now higher than at any point in the last 800,000 years and says that stringent methane restrictions are vital to curbing global warming. CO2 concentrations are unprecedented in at least the last 2 million years.
    • Strong, rapid and sustained reductions in methane emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality.

    Note: Aerosols such as sulphur dioxide (SO2) and nitrous oxides (NO2) are responsible for air pollution, often found in concentrated levels in cities, for example. They cause 4.2 million premature deaths per year, but also temporarily bring a cooling effect to the atmosphere.

    • Ending aerosol pollution would have health and finance benefits, but its masking effect on global warming will lift. Reducing methane emissions will also offer a strong co-benefit of balancing out the warming that will come when this masking effect ends.
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