Introduction

We know that the world will get warmer and the sea level will continue to rise unless we intervene and reduce our CO₂ emissions to zero. We have been adding this ancient CO₂ to the atmosphere in prodigious amounts at rates far in excess of those seen in recent past geological history. We estimated the rate of change of increased CO₂ levels based on known emissions. At the present time, we are emitting about 40 Gt of CO₂ per year. This alone causes about a 3+ ppm increase in atmospheric CO₂ per year. If we project a similar pattern of increasing CO₂ excess into the atmosphere, we will reach over 650 ppm CO2 by 2100. That will cause an unacceptable rise in temperature and sea level as well as all the many impacts that accompany that extreme situation.

The sensible thing to do is eliminate all excess CO₂ emissions and other greenhioouse gas emissions as well. That is an enormous task, so we might be smart investigatge the potential to try something less drastic.

Background

But first some background informaiotn. We know from the geological record that during the Miocene, the CO₂ rose from about 400ppm to 500ppm. It is difficult to be completely certain of the average global temperature at that time, but it is extimated that the temperature rose about 4 0 C during that time. The beginning temperature was warmer than today, perhaps at about 4-5⁰ C. That meaans the temperature at 500ppm CO₂ probably rose to about 8-9⁰C as a global average.

The rate of increase of atmospheric temperature under current conditions has been highly irregular on the scale of years. But using the rate of change of global average temperature over the last century, we estimate that the temperature anomaly will reach almost 4⁰C by 2100. If we continue this trend, we will reach 8⁰C by 2200, and 14⁰C by 2300 unless we take significant action to change the current rate of CO₂ emissions. We must not ignore the chaos-induced, built-in limits on our ability to make accurate predictions based on our statistical treatments of empirically-derived data. Near-term faster impacts are important for our ability to adapt, but do not change the long-term impacts.

Assuming a smooth response to temperature, and at the rate of 3 m/⁰C/3,000 years accumulating each year, and further assuming a capped emissions attitude (that is, planning no increase or decrease from the 2017 rate of adding CO2 to the atmosphere), the sea level rise is estimated to be about 0.5 m by 2050, 1.0 m by 2100, 2.8 m by 2200, and 6.1 m by 2300.

Intervention Scenario #1

Capped Emissions at 2017 Levels

Suppose as a first level of intervention, the world agrees to cap the emission of greenhouse gases at the presnt level. Using CO₂ as our indicator of what is going to happen, the amount of CO₂ remains at about 405ppm.

Although we have stopped increasing the rate of emissions of greenhouse gases, we are nonetheless adding CO₂ to the atmosphere at a constant rate.If we were to coninue to do that for the enxt 200 years, the atmospheric concentration of CO₂ would be in excess of 1,200ppm. That would ultimately be sdisastrous for everyone on the planet.

Under these conditions, the temperature would also continue to rise. By the end of the same 200 years, the temperature on average for the planet would probably have risen to about 8⁰C - a temperature that would be very difficult for most people on the planet .

During the 50 year period of slowing our emissions to zero, we would of course continue to add to the greenhouse gases in the atmosphere until we are actually at zero. In fact, CO₂ would most likely continue to increase to about 520ppm.Once the concentration reached that level, and with no further additions, the CO₂ level would likely remain at that level for thousands of years because it takes a very long time for CO₂ to be absorbed by the ocean, the land, and by plants - especially because they will already be somewhat overloaded.

The result of reaching and holding the atmospheric concentration of CO₂ at about 520ppm would place the planet a little warmer than the warmest of the warmest of the Mid Miocene Climate Optimum - roughly 11.5⁰C as a global average temperature.The temperature would not be reached overnight, but would probably take between 150 to 200 years to reach that level. By then the tropics would likely be too hot for most mammals, including humans.

Given that the temperature would reach over 11⁰C, the sea level would also likely eventually reach about about 35m above the current levels. That would destroy all the coastal infrastructure. The time frame is very long,so there would be time to prepare for the loss which would be complete probably by about 3200AD. Even so, the neeed to confront very high temperatures, complete loss of coastal infrastructure, increased drought, increased storm intensity, and the need to move literally billions of people will be challenging at best, and ultimately disastrous at worst, if humans cannot cooperate, and instead turn to conflict.

350ppm CO₂ Scenario

One of the most aggressive scenarios being suggested today is that the best target for our climate would be to reduce the atmospheric concentration of CO₂ to 350ppm. We don't know how best to reduce greenhouse gases in the atmosphere, but we do know some techniques. Let's assume we can successfully cease all excess emissions and reduce the greenhouse gases. Let's also say we can do this by 2100. The CO₂ in the atmosphere would spike at about 520ppm, then drop rapidly to about 350ppm.

This feels like a really fine option. But remember what the temperature and sea level was like at 300ppm CO₂. It was about 2⁰C warmer with about 6-9m above today's sea level. So 350ppm is going to be pretty bad. How bad? Well the temperature will increase by about 4 to 5⁰C (2100AD) and the sea level would rise about 14m eventually (~3000AD).

The Ultimate Scenario - 280 ppm CO₂

We know from the recent geological record and proxy measurements thathte pre-industrial levels of CO₂ was about 280ppm. We also know that at 280ppm the temperature and sea level were at commfortable levels. This is often seen as the optimum temperature and sea level for human civilization. We do not yet have the knowledge of exactly how best to reduce the atmospheric CO₂, but assuming we can do this, the CO₂ would again spike to about 520ppm then fall to 280ppm.The temperature would increase by about 4⁰C. This time, instead of the temperature remaining stable, it would slowly drop back to a near-zero anomaly. The time to reach that equilibrium would depend on the rate of cooling which in the past ranged from about 4 to 11 times longer than the warm-up. The warm-up time was about 250 to 300 years. That means the cool-down period could be as short as 1,000 years or as long as 3,300 years.

We do not yet have the knowledge of exactly how best to reduce the atmospheric CO₂, but assuming we can do this, the CO₂ would again spike to about 520ppm then fall to 280ppm.

The temperature would increase by about 4⁰C. This time, instead of the temperature remaining stable, it would slowly drop back to a near-zero anomaly. The time to reach that equilibrium would depend on the rate of cooling which in the past ranged from about 4 to 11 times longer than the warm-up. The warm-up time was about 250 to 300 years. That means the cool-down period could be as short as 1,000 years or as long as 3,300 years. The graph depicts a line at an assumptioin of 6 times cooling period and a shaded area of uncertainty where the shortest and longest times are illustrated.