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\subsection{Normal Ranges}\label{ss-normal-range}
The Earth's life-support system is a dynamic system and changes are continuously taking place at a wide range of spatial and temporal scales. However, there are many biological, chemical and physical feedback processes and loops that create prolonged periods of homeostatic stability, in which the system exhibits relatively small variations around a surprisingly stable mean state. For the last one million years, the climate system has fluctuated between cold glacial periods (ice ages) and warm inter-glacial periods, with the transition from warm to cold periods often taking much longer than the reverse transition. Data gathered from many natural recorders of climate variability, e.g., tree rings, ice cores, fossil pollen, ocean sediments, coral and historical data, has been used to reconstruct many variables of the climate system and the overall planetary system. This so-called paleo-data can be used to establish a baseline indicating the range in which these variables have fluctuated during the last approximately one million years. This baseline provides a basis for the assessment of the syndrome of modern global change.
An essential variable associated with the planetary energy state is the Earth's energy imbalance. This is the difference between incoming solar energy and outgoing energy. Among others, the imbalance results from solar energy being used to produce biomaterial that partly is being deposited in subsurface reservoirs as fossil fuels. The energy used by non-human species, including plants, to generate and modify flows is at a very low level compared to the incoming solar energy. As a result, averaged over the last 200 million years, the imbalance was roughly on the order of $10^{-10}$, that is, on the order of 10 MegaWatt of the incoming roughly 98,000 TeraWatt of solar energy were not returned to space. On shorter time scales, the energy imbalance varied between much larger positive and negative values particularly during the cooling or warming of ocean waters and the aggregation and melting of large ice sheets.
An essential variable indicating the stability of the biological system is the rate at which species go extinct. Although it is difficult to estimate background rates for extinction \cite[]{pimm++2014,ceballos++2015}, it appears that outside the five known periods of mass extinction \cite[e.g.,][]{bond+gra2017,starr2018} the extinction rates are very small. For mammals, a recent study estimates the background rate to be 2 extinctions per 10,000 species per 100 years \cite[][ and the reference therein]{ceballos++2015}. However, during the transitions between the glacial and inter-glacial periods in the last one million years, extinction rates appear to have been higher. For example, terrestrial ecosystems experienced major transformations during the most recent warming and associated climate change after the last ice age, and many plant species went extinct \cite[]{nolan++2018}.
For the climate system, a number of physical and chemical variables are essential \cite[]{bojinski++2014}. Table~\ref{t-baseline} gives the ranges for a few essential climate variables for the last 800,000 years and for the Holocene. The total ranges for the greenhouse gases $\mathrm{CO_2}$ and $\mathrm{CH_4}$ are 130 ppm and 480 ppb, respectively, translating into a total range of the greenhouse gas forcing of 4 W/m$^2$, which is similar to the total range of albedo forcing. Using the ranges in Table~\ref{t-baseline}, 1 ppm change in atmospheric $\mathrm{CO_2}$ corresponds to a change of 1 m in global mean sea level, and ~25 ppm correspond to 1\degree C in global mean air temperature.
\begin{table}
\caption{\label{t-baseline}``Normal range'' of selected essential climate variables.
The normal ranges for the selected variables are determined based on data provided by \cite{hansen++2008} for the last 800,000 years. The long-term range is for the last 800,000 years prior to industrialization. The range for the Holocene is for the last 12,000 years before industrialization. Current values are for 2017. The projected ranges are those published based on model studies.}
\bc
{\small
\begin{tabular}{lcccc}\hline
{\bf Variable} & {\bf long-term range} & {\bf Holocene} & Current & projected 2100 \\ \hline
$\mathrm{CO_2}$ & 170-300 ppm & 270-285 ppm & 405 ppm & 500-900 ppm \\
$\mathrm{CH_4}$ & 320-800 ppb & 550-800 ppb & 1848 ppb & 2000-2500 ppb \\
Greenhouse gas forcing & -4.0 - 0.0 W/m$^2$ & -0.3 - 0.0 W/m$^2$ & 1.0 W/m$^2$ & 3.5 - 5.0 W/m$^2$ \\
Albedo forcing & -4.0 - 0.0 W/m$^2$ & -0.2 - 0.0 W/m$^2$ & 0.3 W/m$^2$ & 1.5 - 2.5 W/m$^2$ \\
Global mean surface temperature & -4.0 - 1.0\degree C & 0.0 - 1.0\degree C & 1.0\degree C & 2 - 6\degree C \\
Global mean sea level & -130 - 5 m & -3 - 0.0 m$^*$ & 0.3 m & 1 - 5 m \\ \hline
\end{tabular}
}
\ec
{\footnotesize $^*$This is for the last 7,000 years during the time sea-level was exceptionally stable.}
\end{table}
During the Holocene, the range for the greenhouse gases was only a fraction of the range during the last 800,000. Likewise, global mean temperature variations were limited to a range of 1\degree C. During the first part of the Holocene, sea level continued to rise nearly 50 m until an equilibrium between the air temperature and land-based ice masses was reached about 7,000 years ago. Since then, the range of global sea level change was only on the order of a few meters.
Your Comments and Questions
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