We live on the one-and-only habitable planet in the solar system, Earth (Figure 1). On Mars the average near- surface air temperature is –65˚C (–85˚F), the surface pressure is 1/160 that of Earth, and the atmosphere is nearly all carbon dioxide [1] . On Venus the average near-surface air temperature is 464˚C (867˚F), the surface pressure is 92 times that of Earth, and the atmosphere is nearly all carbon dioxide [1] . The other planets in the solar system are no more hospitable, with either no atmosphere or an atmosphere that is toxic to life.

Astronomers have found several other planets within the “Goldilocks Zone” of other stars, the zone wherein radiation from the star is equal to that at Venus from the Sun—about twice that at Earth—to that at Mars—about half of that at Earth. While that is necessary for a planet to have liquid water, and thus life as we know it, it is not sufficient. Neither Venus nor Mars has surface liquid water or life. Why? Because Venus has too much carbon dioxide, and is thus too hot, and Mars has too little carbon dioxide, and is thus too cold. For liquid water and life to exist, a planet must not only be in the Goldilocks zone, it must also have the right amount of greenhouse gases. Earth is the one-and-only planet in the solar system that has the right amount of greenhouse gases to permit life.

The distance to the nearest star other than the Sun, Proxima Centauri, is 4.2 light years (40 trillion kilometers = 25 trillion miles) [2] (Figure 2). Our space probe Voyager 1, launched in 1977, has left the solar system and is traveling at 35,000 miles an hour [3] . At that speed it would take 80,000 years to reach Proxima Centauri [2] . So, even if Proxima Centauri has an Earth-like planet, it is too far away for us to reach it. Thus Earth is very likely the only planet for an enormous distance that permits life, for at least 80,000 years’ travel time at the speed of our space probes. Accordingly our home planet truly is “This Island Earth” [4] .

We have published three papers on how we can safeguard the Earth’s climate [5] -[7] . These papers are described in the following section, as is how this paper differs from them.

In our three antecedent papers we used our Simple (engineering-type) Climate Model [8] to calculate the change in global-average near-surface air temperature from 1765 through year 3000 for historical emissions and two scenarios of future emissions of greenhouse gases (GHGs): (1) a Reference scenario—the Representative Concentration Pathway 8.5 scenario (RCP-8.5) and its extension (ECP-8.5) from 2100 through 2500 [9] —thereafter the emissions were kept constant through 3000; and (2) a Mitigation scenario—the Fair Plan scenario.

The Fair Plan is fair in two ways. First, it uses trade-adjusted emissions, that is, the GHG emissions incurred by country E in producing goods and services imported therefrom by country I are considered as the GHG emissions of country I, not country E. Second, the intensity of GHG emissions [10] of the developed (so-called Annex B or AB) countries and developing (non-Annex B or nAB) countries are decreased differently from unity

starting in year YS to zero in year YE such that: (1) the cumulative GHG emissions for AB and nAB countries are equal, and (2) the maximum increase in global-mean near-surface air temperature does not exceed the 2˚C (3.6˚F) limit adopted by the United Nations Framework Convention on Climate Change (UNFCCC) “to prevent dangerous anthropogenic interference with the climate system” [11] [12] . The GHG intensity decreases from unity in YS to zero in YE linearly for AB countries and more slowly at first for nAB countries, thereby allowing their continuing economic development.

In our first Fair Plan paper (FP1) [5] we prescribed YS = 2015 and YE = 2100, that is a phase-out duration D = YE – YS = 85 years. In FP2 [6] we examined the effects of increasing YS from 2015 to 2030 in 5-year intervals, and D from 50 to 100 years in 10-year intervals. We found that phase-out during 2020-2100 is optimal. In FP3 [7] we added to the human-caused temperature changes through 2100 the natural temperature changes caused by three quasi-periodic oscillations that are predictable on a year-to-year basis by sine waves, and random variations that are not predictable on a year-to-year basis, but are represented by their 90% confidence interval. In this our fourth Fair Plan paper we do not consider these natural temperature variations, and we again examine the entire millennium.

To simulate the change in global-mean near-surface air temperature, ∆T_s, with our Simple Climate Model, the climate sensitivity, ∆T_2x, must be specified. ∆T_2x is the change in the equilibrium (net incoming radiation at the top of Earth’s atmosphere R = 0) ∆T_s due to the radiative forcing resulting from a doubling of the preindustrial CO₂ concentration (R = 3.71 Wm^–2). We have determined ∆T_2x from the observed ∆T_s compiled by four research groups: (1) Hadley Centre/Climate Research Unit (∆T_2x = 1.61˚C); (2) NASA (∆T_2x = 1.45˚C); (3) NOAA (∆T_2x = 1.99˚C); and (4) Japan Meteorological Agency (∆T_2x = 2.01˚C) [13] . In FP1 we presented ∆T_s individually for the ∆T_2x values of (1) - (3), while in FP2 and FP3 we did so individually for all four ∆T_2x values. Here we combine those four ∆T_s values into an expected value for ∆T_s, this to simplify and accentuate the message of FP1 - FP3. The expected value of ∆T_s is taken as the average of the four ∆T_s values.

The CO₂ emissions for the Reference and Fair Plan scenarios are shown in Figure 3. In the Reference scenario the CO₂ emission rises from about 10 billion tonnes of carbon per year (GtC/year) in 2010 to 29 GtC/year in 2100, remains essentially constant until 2150, and then decreases to 2 GtC/year in 2300, where it remains essentially constant throughout the remainder of the millennium. The CO₂ emissions for the Fair Plan scenario are the product of the CO₂ emissions for the Reference scenario and the intensity of GHG emissions shown in Figure 4. These emissions rise from about 10 GtC/year in 2010 to 13.5 GtC/year in 2046, and then decrease to zero in 2100. For the emissions of the other GHGs we took the curves from RCP-8.5 and applied the same intensities for AB and nAB as we did for CO₂.

The resulting CO₂ concentrations are shown in Figure 5. For the Reference scenario the CO₂ concentration increases to 1928 parts per million by volume (ppmv) in 2237—more than six times the pre-industrial concentration of 277 ppmv—and then slowly decreases to about 1750 ppmv by the end of the millennium due to natural carbon sinks. For the Fair Plan scenario the CO₂ concentration increases to 612 ppmv in year 2087—just over twice the pre-industrial concentration—and then decreases to 406 ppmv by the end of the millennium.

The expected value of the change in global-average near-surface air temperature from 1765 for the historical greenhouse-gas emissions (black curve) and the Reference (red curve) and Fair Plan (green curve) emission scenarios are shown in Figure 6. The UNFCCC limit of 2˚C (3.6˚F) “to prevent dangerous anthropogenic interference with the climate system” is shown by the dashed grey line.

The temperature change for the Reference scenario increases to 5.2˚C (9.4˚F) in 2225 and remains essentially constant through the remainder of the millennium, that is, for 40 human generations. The temperature change for the Fair Plan scenario peaks very near the 2˚C (3.6˚F) UNFCCC limit in 2082 and decreases through the remainder of the millennium.

Because there is nowhere else for humanity to live within at least 40 trillion kilometers (25 trillion miles)—a distance that would take us 80,000 years to reach at the velocity of Voyager 1—we live on “This Island Earth”. If humanity does not abate its emissions of greenhouse gases, the expected value of global-mean near-surface air temperature will increase by 5.2˚C (9.4˚F) for at least 40 human generations. If we phase out our emissions of greenhouse gases from 2020 to 2100 following our Fair Plan, the expected value of the increase in global-mean near-surface air temperature peaks very near the 2˚C (3.6˚F) limit adopted by the UNFCCC “to prevent dangerous anthropogenic interference with the climate system”. Accordingly we must safeguard the climate of “This Island Earth”.