Socio Pathways

To improve the accuracy of future predictions, scientists developed the Representative Concentration Pathways (RCPs). A Representative Concentration Pathway is a greenhouse gas concentration (not emissions) trajectory adopted by the International Panel on Climate Change to classify the stringency of different warming limits. Four pathways were used for climate modeling and research for the IPCC fifth Assessment Report (AR5) in 2014. The pathways describe different climate futures, all of which are considered possible depending on the volume of greenhouse gases emitted in the years to come. The Representative Concentration Pathway values refer to the radiative forcing from the sun expressed in watts per square meter (W/m2) by the end of the century compared to preindustrial times. Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing solar energy in the earth-atmosphere system. 

The nations participating in the Paris Agreement have agreed that they will commit themselves to prevent global temperature to increase 2 °C above pre-industrial levels. Scientists from the International Panel on Climate Change have calculated that to achieve this goal, the radiative forcing of our atmosphere should not exceed 2.6 watts per square meter by the year 2100. In other words, this corresponds with Representative Concentration Pathway 2.6 (RCP2.6). This radiative forcing of 2.6 watts per square meter predicts a globally maximum concentration of 440 particles of greenhouse gases per million near the year 2050. After this period the concentration will slowly decline. If we want to prevent global temperature to increase 1.5 °C above pre-industrial levels, this would correspond with Representative Concentration Pathway 1.9 (RCP1.9). To reach this goal the concentration of greenhouse gases in our atmosphere should be below 393 part per million by the year 2100. Remember that currently in 2020 we’ve already reached a day with 416 ppm of CO2 in the air.

Remember also that our first calculation (chapter 4.2) gave us a total carbon budget that only lasted for the coming 38 years. New calculations including Representative Concentration Pathways, Transient Climate Response and other methods only leave us with a maximal carbon budget for the next 34 years and some even with just 11 years, depending on which method is used! It is important to keep in mind that the Representative Concentration Pathways only focus on concentrations of greenhouse gases they do not include the effects of earths carbon circle. With proper use of earth’s natural sinks we will still be able to extract CO2 from the atmosphere. This will be discussed later on. For now the urge and uncertainties involving climate change make it important to develop broad climate prediction schemes that include human activities. 

V- Representative Concentration Pathways (RCP)

For classifying the stringency of different warming limits the concept of Representative Concentration Pathways (RCPs) has been introduced to climate change research.
They are called “Pathways” in the literature, but in fact, constitute projections of greenhouse gas emissions and concentrations and their combined radiative forcing.

“Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing solar energy in the earth-atmosphere system. It is an index of the importance of the factor as a potential climate change mechanism. In this book, radiative forcing values are for changes relative to preindustrial conditions defined in the year 1750 and are expressed in watts per square meter (W/m2).” The RCPs prescribe levels of radiative forcing (W/m2) arising from different atmospheric concentrations of GHGs that lead to different levels of climate change by the year 2100. For example, RCP2.6 (2.6 W/m2) is projected to lead to global mean temperature changes of about 0.9°C–2.3°C, and RCP8.5 (8.5 W/m2) to global mean temperature changes of about 3.2°C–5.4°C at the year 2100 (van Vuuren et al. 2014).

The original RCPs comprised four projections, ranging from RCP2.6 to RCP8.5. For comparison, 3.7 Watt/m2 stands for a doubling of atmospheric CO2 concentration from preindustrial times, i.e. from ca. 280 parts per million (ppm) air molecules to a 560 ppm!
In the context of climate change, the term “forcing” is restricted to changes in the radiation balance between receiving, absorbing and reflecting the energy from the sun within earth’s surface-troposphere. This forcing is imposed by external factors like greenhouse gasses, with no changes in stratospheric dynamics, and no surface and tropospheric feedbacks in operation. It is important to keep in mind that RCPs do not consider the influence of earth’s CO2 sinks (can) have on the amount of CO2 that stays in the atmosphere.

VI- Transient Climate Response

The Transient Climate Response to cumulative Emissions of carbon (TCRE) takes into account the unique characteristics of the Earth system’s response to anthropogenic carbon emissions. The TCRE is the ratio of the globally averaged surface temperature change per unit carbon dioxide (CO2) emitted. This response is conceived as the amount that global temperature changes per the amount of total carbon dioxide in the atmosphere taking in consideration the atmospheric lifetime of CO2. With respect to cumulative CO2 emissions over time, global temperature is reasonably estimated to change linearly regardless of the path taken to reach peak CO2 emissions. This means that for specific amount of cumulative CO2 emissions, a known global temperature change (within a range of uncertainty) can be expected. TCRE is denned as global average surface temperature change per unit of total cumulative anthropogenic CO2 emissions, typically 1,000PgC, or 1 trillion ton of carbon. In AR5, the TCRE was assessed to be ‘likely’ to lie (that is, with greater than 66% probability) between 0.8 to 2.5C per 1,000 PgC for cumulative CO2 emissions less than about 2,000PgC and until the time at which temperature peaks. The constancy of TCRE means that it can also be assessed for the real world by dividing an estimate of CO2-induced warming to date by an estimate of anthropogenic CO2 emissions. Such an approach relies on a calculation of greenhouse gas-attributable warming using a regression of observed warming onto the simulated response to greenhouse gases and other forcing’s, and an estimate of the ratio of CO2 to total greenhouse gas radioactive forcing or temperature response.

Alternatively, TCRE may be assessed from observations by applying observational constraints to the parameters of a simple carbon-cycle climate model and evaluating the ratio of warming to emissions for the constrained model. For a carbon budget approach to make sense, TCRE must be reasonably independent of the pathway of emissions. Earlier studies have indeed shown that this is the case, at least for peak warming and monotonously increasing cumulative carbon emissions. If a set carbon budget limit is exceeded, CO2 needs to be actively removed from the atmosphere afterward.
For scenarios that limit end-of-century warming to below 2°C with a likely probability, the WGIII (IPCC Working Group III) assessment reports the following: “For limiting warming the earth to below 2°C relative to pre-industrial levels with greater than 66% probability, the remaining CO2 budget from 2015 onwards for CO2-induced warming only is 1,610 GtCO2”.

When considering TABs until peak warming, based on the stringent mitigation scenarios of the IPCC AR5 Scenario Database, a range of 590– 1,240 GtCO2 is found for limiting warming to below 2°C with >66% probability from 2015 onwards. Variations within this range depending on the probability of staying below 2 °C and on end-of-century non-CO2 warming. Current CO2 emissions are about 36 GtCO2yr -1, and global CO2 emissions thus have to be reduced urgently to keep within a 2 °C-compatible budget. At the current rate of 36 GTCO2yr we have between 590/36 till 1240/36 years to reach this maximum amount of carbon budget. This is roughly between 16 to 34 years.

• Carbon budget calculations based on max 450 ppm CO2
A total atmospheric carbon budget of 1 trillion equals 3667 GtCO2. Remember that each 1 ppm of CO2 in the atmosphere equals around 2.13 GtC or 7.81 GtCO2. This means that a carbon budget of 1 trillion ton equals a density of 469.5 CO2 per million particles in the atmosphere. The other way around; If the amount of CO2 in the atmosphere shouldn’t be more than 450 ppm by the year 2100, then this corresponds with a total amount of 450×2.13 is 959 Gigatonne of carbon and 3515 GtCO2 in the atmosphere… The highest CO2 ppm of 2019 was 414.7 in May. This means that there was 883.3 Gton of carbon and 3239 GtCO2 in the atmosphere. If we take 3239 from 3515, then our net ‘carbon budget’ comes at only 276 GtCO2. Divided by our current yearly CO2 emission of 36 GtCO2 we have only 8 years left before we reach earth’s possible carbon budget! Luckily the earth still absorbs about 40 to 50% of this emission (around 17 GtCO2), which still leaves us with a budget of just over 14 years. At the following websites, you can find the latest official IPCC Carbon report from 2018 and studies from other scientific reports: IPCC.ch and PBL.nl.