Why 1.5ºC?The Paris Agreement goal is to reach less than 1.5º C. It completes by the year 2100. The goal's target is to allow Earth to warm to about 1.5ºC (2.7ºF) above the average temperature from 1850-1900. Also note, 1.5ºC is the global average: locally your town or city could warm greater than 1.5º C; it could warm more than of 10º C (or 18º F, like during the summer of 2010: "[...] anomalies were particularly pronounced, exceeding the 1970–1999 mean (18) by 10°C [... ]".) And lastly, the goal follows (economically) viable trajectory to 1.5 that includes exceeding 1.5ºC before returning to below 1.5ºC at the end of the century.
The section of my previous post, Climate Action to Carbon Removal Q&A: "Why is the amount of Carbon so much larger ..." is incorrect. The climate modeling publicly available referenced in the Nature Article: 'Scenarios towards limiting global mean temperature increase below 1.5 °C' shows the climate returns to that of the mid-2020s or the 2000s in another graph. Because of this difference, I can't use the historical projection of radiative forcing to project the target climate. The climate of 2100 from SSPn-1.9 is more likely comparable to a climate similar to mid-2000 or mid-2020s and includes all climate extremes for those decades. Not to mention that the temperature will still rise from industrial use mid-century before finally descending to about 1.5ºC-1.4ºC. And it's worth underscoring, 1.5º C is relative to the starting timeframe of 1850-1900, per Special Report 1.5.
Figure 1. Climate Models of SSP1 1.9 IMAGE modeled by MAGICC in Green and FAIR in Purple by the IAMC Scenario Explorer hosted by IIASA
So why is the amount still so much more vast to reach 280 ppm in the atmosphere and oceans compared to only reaching a little below 1.5º C?Think back to the original problem, we emitted Carbon Dioxide, which has accumulated in the atmosphere and oceans since the start of industry (1750). The accumulation of Carbon is roughly 460 gigatonnes of Carbon (1.8 trillion tonnes CO₂) or 80 thousand Great Pyramids of Giza, or 1.5M Empire State buildings, spread between the atmosphere and oceans. To restore the climate to a desired Carbon Dioxide concentration, we need to remove Carbon Dioxide we added that now resides in the atmosphere and oceans. We emitted about 360 gigatonnes of Carbon (1.3 trillion tonnes of CO₂) from 1750 to 2010. The target scenarios would remove on average 100 gigatonnes of Carbon (366 gigatonnes of CO₂) which has the potential to return to a climate roughly similar to 2010. Given the changes to the other greenhouse gases, the MAGICC model in Figure 1. shows a temperature more similar to what's projected for the mid-2020s. The amount to return to pre-industrial times would be the entire amount of Carbon we've emitted, all 460 gigatonnes of Carbon. This difference is perceived as politically untenable as it's perceived to be too expensive (with today's pricing of renewables and need for fossil fuels). If we wish to have the climate of say 300 ppm, we'd have to remove another 300 gigatonnes (≈1 trillion tonnes CO₂) beyond what it would take to reach 1.5ºC. To restore to a desired temperature in addition to removing CO₂, we would also need to draw down much of the short forming greenhouse gasses.
Figure 2. Cumulative Emissions are a super-exponential inducing changes to the natural sinks. SSP1 1.9 IMAGE, as well as SSP1 RCP2.6 IMAGE, are modeled for comparison. The X's listed in red are the amount of CO₂ it would take for the Atmosphere to reach 450 ppm and extrapolated to the Cumulative Emissions line. We are likely to deglaciate Antarctica when we hit 450 ppm. The SSPs splines were generated from a linear extrapolation of the SSP Database data.
No doctor would ever say to a cancer patient, 'We might be able to treat your cancer by providing the least amount of care and intervention.' That's exactly the target goal 1.5ºC primarily due to political will based on public support. It's from the support my government has historically supplied. And for that, I apologize to the world.
The UN Climate Action Summit 2019 theme referring to the climate emergency, was: 'A Race We Can Win. A Race We Must Win.' We should seek much lower than 1.5ºC should we want to secure an environment that fairly provides economic justice to all life on Earth. We didn't save the condors, bald eagles, Asian elephants, black-footed ferrets, kakapos, manatees, orangutans, the spotted owls, giant pandas, tigers, the leatherback sea turtles, the great blue whales, the humpback whales, and the redwoods, thousands square meters of rain forests, only to be lost again in another 60-100 years. We didn't decide we wanted to have Sustainable Development Goals only for the G7. A world on the brink of peril, say one created from vast amounts of CO₂; about 360 gigatonnes of Carbon (and 1.3 trillion tonnes of CO₂) similar to 2010 or that of the mid-2020s, isn't one social-ecological and economic justice.
And why? Because we're too lazy to change an existing infrastructure that delivers fossil fuels that creates wealth, and renewables are too expensive today? Those reasons aren't good enough to risk Earth for centuries to come.