Archive for the ‘Impacts’ Category

In one of the iconic climate model tests, CO2 is doubled from a pre-industrial level of 280ppm to 560ppm “overnight” and we find the new steady state surface temperature. The change in CO2 is an input to the climate model, also known as a “forcing” because it is from outside. That is, humans create more CO2 from generating electricity, driving automobiles and other activities – this affects the climate and the climate responds.

These experiments with simple climate models were first done with 1d radiative-convective models in the 1960s. For example, Manabe & Wetherald 1967 who found a 2.3ºC surface temperature increase with constant relative humidity and 1.3ºC with constant absolute humidity (and for many reasons constant relative humidity seems more likely to be closer to reality than constant absolute humidity).

In other experiments, especially more recently, more more complex GCMs simulate 100 years with the CO2 concentration being gradually increased, in line with projections about future emissions – and we see what happens to temperature with time.

There are also other GHGs (“greenhouse” gases / radiatively-active gases) in the atmosphere that are changing due to human activity – especially methane (CH4) and nitrous oxide (N2O). And of course, the most important GHG is water vapor, but changes in water vapor concentration are a climate feedback – that is, changes in water vapor result from temperature (and circulation) changes.

And there are aerosols, some internally generated within the climate and others emitted by human activity. These also affect the climate in a number of ways.

We don’t know what future anthropogenic emissions will be. What will humans do? Build lots more coal-fire power stations to meet energy demand of the future? Run the entire world’s power grid from wind and solar by 2040? Finally invent practical nuclear fusion? How many people will there be?

So for this we need some scenarios of future human activity (note 1).

Scenarios – SRES and RCP

SRES was published in 2000:

In response to a 1994 evaluation of the earlier IPCC IS92 emissions scenarios, the 1996 Plenary of the IPCC requested this Special Report on Emissions Scenarios (SRES) (see Appendix I for the Terms of Reference). This report was accepted by the Working Group III (WGIII) plenary session in March 2000. The long-term nature and uncertainty of climate change and its driving forces require scenarios that extend to the end of the 21st century. This Report describes the new scenarios and how they were developed.

The SRES scenarios cover a wide range of the main driving forces of future emissions, from demographic to technological and economic developments. As required by the Terms of Reference, none of the scenarios in the set includes any future policies that explicitly address climate change, although all scenarios necessarily encompass various policies of other types.

The set of SRES emissions scenarios is based on an extensive assessment of the literature, six alternative modeling approaches, and an “open process” that solicited wide participation and feedback from many groups and individuals. The SRES scenarios include the range of emissions of all relevant species of greenhouse gases (GHGs) and sulfur and their driving forces..

..A set of scenarios was developed to represent the range of driving forces and emissions in the scenario literature so as to reflect current understanding and knowledge about underlying uncertainties. They exclude only outlying “surprise” or “disaster” scenarios in the literature. Any scenario necessarily includes subjective elements and is open to various interpretations. Preferences for the scenarios presented here vary among users. No judgment is offered in this Report as to the preference for any of the scenarios and they are not assigned probabilities of occurrence, neither must they be interpreted as policy recommendations..

..By 2100 the world will have changed in ways that are difficult to imagine – as difficult as it would have been at the end of the 19th century to imagine the changes of the 100 years since. Each storyline assumes a distinctly different direction for future developments, such that the four storylines differ in increasingly irreversible ways. Together they describe divergent futures that encompass a significant portion of the underlying uncertainties in the main driving forces. They cover a wide range of key “future” characteristics such as demographic change, economic development, and technological change. For this reason, their plausibility or feasibility should not be considered solely on the basis of an extrapolation of current economic, technological, and social trends.

The RCPs were in part a new version of the same idea as SRES and published in 2011. My understanding is that the Representative Concentration Pathways worked more towards final values of radiative forcing in 2100 that were considered in the modeling literature, and you can see this in the names of each RCP.

from A special issue on the RCPs, van Vuuren et al (2011)

By design, the RCPs, as a set, cover the range of radiative forcing levels examined in the open literature and contain relevant information for climate model runs.

[Emphasis added]

From The representative concentration pathways: an overview, van Vuuren et al (2011)

This paper summarizes the development process and main characteristics of the Representative Concentration Pathways (RCPs), a set of four new pathways developed for the climate modeling community as a basis for long-term and near-term modeling experiments.

The four RCPs together span the range of year 2100 radiative forcing values found in the open literature, i.e. from 2.6 to 8.5 W/m². The RCPs are the product of an innovative collaboration between integrated assessment modelers, climate modelers, terrestrial ecosystem modelers and emission inventory experts. The resulting product forms a comprehensive data set with high spatial and sectoral resolutions for the period extending to 2100..

..The RCPs are named according to radiative forcing target level for 2100. The radiative forcing estimates are based on the forcing of greenhouse gases and other forcing agents. The four selected RCPs were considered to be representative of the literature, and included one mitigation scenario leading to a very low forcing level (RCP2.6), two medium stabilization scenarios (RCP4.5/RCP6) and one very high baseline emission scenarios (RCP8.5).

Here are some graphs from the RCP introduction paper:

Population and GDP scenarios:


Figure 1 – Click to expand

I was surprised by the population graph for RCP 8.5 and 6 (similar scenarios are generated in SRES). From reading various sources (but not diving into any detailed literature) I understood that the consensus was for population to peak mid-century at around 9bn people and then reduce back to something like 7-8bn people by the end of the century. This is because all countries that have experienced rising incomes have significantly reduced average fertility rates.

Here is Angus Deaton, in his fascinating and accessible book for people interested in The Great Escape as he calls it (that is, our escape from poverty and poor health):

In Africa in 1950, each woman could expect to give birth to 6.6 children; by 2000, that number had fallen to 5.1, and the UN estimates that it is 4.4 today. In Asia as well as in Latin America and the Caribbean, the decline has been even larger, from 6 children to just over 2..

The annual rate of growth of the world’s population, which reached 2.2% in 1960, was only half of that in 2011.

The GDP graph on the right (above) is lacking a definition. From the other papers covering the scenarios I understand it to be total world GDP in US$ trillions (at 2000 values, i.e. adjusted for inflation), although the numbers don’t seem to align exactly.

Energy consumption for the different scenarios:

Figure 2 – Click to expand

Annual emissions:

Figure 3 – Click to expand

Resulting concentrations in the atmosphere for CO2, CH4 (methane) and N2O (nitrous oxide):


Figure 4 – Click to expand

Radiative forcing (for explanation of this term, see for example Wonderland, Radiative Forcing and the Rate of Inflation):


Figure 5  – Click to expand

We can see from this figure (fig 5, their fig 10) that the RCP numbers refer to the expected radiative forcing in 2100 – so RCP8.5, often known as the “business as usual” scenario has a radiative forcing, compared to pre-industrial values, of 8.5 W/m². And RCP6 has a radiative forcing in 2100, of 6 W/m².

We can also see from the figure on the right that increases in CO2 are the cause of almost all of most of the increase from current values. For example, only RCP8.5 has a higher methane (CH4) forcing than today.

Business as usual – RCP 8.5 or RCP 6?

I’ve seen RCP8.5 described as “business as usual” but it seems quite an unlikely scenario. Perhaps we need to dive into this scenario more in another article. In the meantime, part of the description from Riahi et al (2011):

The scenario’s storyline describes a heterogeneous world with continuously increasing global population, resulting in a global population of 12 billion by 2100. Per capita income growth is slow and both internationally as well as regionally there is only little convergence between high and low income countries. Global GDP reaches around 250 trillion US2005$ in 2100.

The slow economic development also implies little progress in terms of efficiency. Combined with the high population growth, this leads to high energy demands. Still, international trade in energy and technology is limited and overall rates of technological progress is modest. The inherent emphasis on greater self-sufficiency of individual countries and regions assumed in the scenario implies a reliance on domestically available resources. Resource availability is not necessarily a constraint but easily accessible conventional oil and gas become relatively scarce in comparison to more difficult to harvest unconventional fuels like tar sands or oil shale.

Given the overall slow rate of technological improvements in low-carbon technologies, the future energy system moves toward coal-intensive technology choices with high GHG emissions. Environmental concerns in the A2 world are locally strong, especially in high and medium income regions. Food security is also a major concern, especially in low-income regions and agricultural productivity increases to feed a steadily increasing population.

Compared to the broader integrated assessment literature, the RCP8.5 represents thus a scenario with high global population and intermediate development in terms of total GDP (Fig. 4).

Per capita income, however, stays at comparatively low levels of about 20,000 US $2005 in the long term (2100), which is considerably below the median of the scenario literature. Another important characteristic of the RCP8.5 scenario is its relatively slow improvement in primary energy intensity of 0.5% per year over the course of the century. This trend reflects the storyline assumption of slow technological change. Energy intensity improvement rates are thus well below historical average (about 1% per year between 1940 and 2000). Compared to the scenario literature RCP8.5 depicts thus a relatively conservative business as usual case with low income, high population and high energy demand due to only modest improvements in energy intensity.

When I heard the term “business as usual” I’m sure I wasn’t alone in understanding it like this: the world carries on without adopting serious CO2 limiting policies. That is, no international agreements on CO2 reductions, no carbon pricing, etc. And the world continues on its current trajectory of growth and development. When you look at the last 40 years, it has been quite amazing. Why would growth slow, population not follow the pathway it has followed in all countries that have seen rising prosperity, and why would technological innovation and adoption slow? It would be interesting to see a “business as usual” scenario for emissions, CO2 concentrations and radiative forcing that had a better fit to the name.

RCP 6 seems to be a closer fit than RCP 8.5 to the name “business as usual”.

RCP6 is a climate-policy intervention scenario. That is, without explicit policies designed to reduce emissions, radiative forcing would exceed 6.0 W/m² in the year 2100.

However, the degree of GHG emissions mitigation required over the period 2010 to 2060 is small, particularly compared to RCP4.5 and RCP2.6, but also compared to emissions mitigation requirement subsequent to 2060 in RCP6 (Van Vuuren et al., 2011). The IPCC Fourth Assessment Report classified stabilization scenarios into six categories as shown in Table 1. RCP6 scenario falls into the border between the fifth category and the sixth category.

Its global mean long-term, steady-state equilibrium temperature could be expected to rise 4.9° centigrade, assuming a climate sensitivity of 3.0 and its CO2 equivalent concentration could be 855 ppm (Metz et al. 2007).

Some of the background to RCP 8.5 assumptions is in an earlier paper also by the same lead author – Riahi et al 2007, another freely accessible paper (reference below) which is worth a read, for example:

The task ahead of anticipating the possible developments over a time frame as ‘ridiculously’ long as a century is wrought with difficulties. Particularly, readers of this Journal will have sympathy for the difficulties in trying to capture social and technological changes over such a long time frame. One wonders how Arrhenius’ scenario of the world in 1996 would have looked, perhaps filled with just more of the same of his time—geopolitically, socially, and technologically. Would he have considered that 100 years later:

  • backward and colonially exploited China would be in the process of surpassing the UK’s economic output, eventually even that of all of Europe or the USA?
  • the existence of a highly productive economy within a social welfare state in his home country Sweden would elevate the rural and urban poor to unimaginable levels of personal affluence, consumption, and free time?
  • the complete obsolescence of the dominant technology cluster of the day-coal-fired steam engines?

How he would have factored in the possibility of the emergence of new technologies, especially in view of Lord Kelvin’s sobering ‘conclusion’ of 1895 that “heavier-than-air flying machines are impossible”?

Note on Comments

The Etiquette and About this Blog both explain the commenting policy in this blog. I noted briefly in the Introduction that of course questions about 100 years from now mean some small relaxation of the policy. But, in a large number of previous articles, we have discussed the “greenhouse” effect (just about to death) and so people who question it are welcome to find a relevant article and comment there – for example, The “Greenhouse” Effect Explained in Simple Terms which has many links to related articles. Questions on climate sensitivity, natural variation, and likelihood of projected future temperatures due to emissions are, of course, all still fair game in this series.

But I’ll just delete comments that question the existence of the greenhouse effect. Draconian, no doubt.


Emissions Scenarios, IPCC (2000) – free report

A special issue on the RCPs, Detlef P van Vuuren et al, Climatic Change (2011) – free paper

The representative concentration pathways: an overview, Detlef P van Vuuren et al, Climatic Change (2011) – free paper

RCP4.5: a pathway for stabilization of radiative forcing by 2100, Allison M. Thomson et al, Climatic Change (2011) – free paper

An emission pathway for stabilization at 6 Wm−2 radiative forcing,  Toshihiko Masui et al, Climatic Change (2011) – free paper

RCP 8.5—A scenario of comparatively high greenhouse gas emissions, Keywan Riahi et al, Climatic Change (2011) – free paper

Scenarios of long-term socio-economic and environmental development under climate stabilization, Keywan Riahi et al, Technological Forecasting & Social Change (2007) – free paper

Thermal equilibrium of the atmosphere with a given distribution of relative humidity, S Manabe, RT Wetherald, Journal of the Atmospheric Sciences (1967) – free paper

The Great Escape, Health, Wealth and the Origins of Inequality, Angus Deaton, Princeton University Press (2013) – book


Note 1: Even if we knew future anthropogenic emissions accurately it wouldn’t give us the whole picture. The climate has sources and sinks for CO2 and methane and there is some uncertainty about them, especially how well they will operate in the future. That is, anthropogenic emissions are modified by the feedback of sources and sinks for these emissions.

Read Full Post »

A long time ago, in About this Blog I wrote:

Opinions are often interesting and sometimes entertaining. But what do we learn from opinions? It’s more useful to understand the science behind the subject. What is this particular theory built on? How long has the theory been “established”? What lines of evidence support this theory? What evidence would falsify this theory? What do opposing theories say?

Now I would like to look at impacts of climate change. And so opinions and value judgements are inevitable.

In physics we can say something like “95% of radiation at 667 cm-1 is absorbed within 1m at the surface because of the absorption properties of CO2″ and be judged true or false. It’s a number. It’s an equation. And therefore the result is falsifiable – the essence of science. Perhaps in some cases all the data is not in, or the formula is not yet clear, but this can be noted and accepted. There is evidence in favor or against, or a mix of evidence.

As we build equations into complex climate models, judgements become unavoidable. For example, “convection is modeled as a sub-grid parameterization therefore..”. Where the conclusion following “therefore” is the judgement. We could call it an opinion. We could call it an expert opinion. We could call it science if the result is falsifiable. But it starts to get a bit more “blurry” – at some point we move from a region of settled science to a region of less-settled science.

And once we consider the impacts in 2100 it seems that certainty and falsifiability must be abandoned. “Blurry” is the best case.


Less than a year ago listening to America and the New Global Economy by Timothy Taylor (via audible.com) I remember he said something like “the economic cost of climate change was all lumped into a fat tail – if the temperature change was on the higher side”. Sorry for my inaccurate memory (and the downside of audible.com vs a real book). Well it sparked my interest in another part of the climate journey.

I’ve been reading IPCC Working Group II (wgII) – some of the “TAR” (= third assessment report) from 2001 for background and AR5, the latest IPCC report from 2014. Some of the impacts also show up in Working Group I which is about the physical climate science, and the IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation from 2012, known as SREX (Special Report on Extremes). These are all available at the IPCC website.

The first chapter of the TAR, Working Group II says:

The world community faces many risks from climate change. Clearly it is important to understand the nature of those risks, where natural and human systems are likely to be most vulnerable, and what may be achieved by adaptive responses. To understand better the potential impacts and associated dangers of global climate change, Working Group II of the Intergovernmental Panel on Climate Change (IPCC) offers this Third Assessment Report (TAR) on the state of knowledge concerning the sensitivity, adaptability, and vulnerability of physical, ecological, and social systems to climate change.

A couple of common complaints in the blogosphere that I’ve noticed are:

  • “all the impacts are supposed to be negative but there are a lot of positives from warming”
  • “CO2 will increase plant growth so we’ll be better off”

Within the field of papers and IPCC reports it’s clear that CO2 increasing plant growth is not ignored. Likewise, there are expected to be winners and losers (often, but definitely not exclusively, geographically distributed), even though the IPCC summarizes the expected overall effect as negative.

Of course, there is a highly entertaining field of “recycled press releases about the imminent catastrophe of climate change” which I’m sure ignores any positives or tradeoffs. Even in what could charitably be called “respected media outlets” there seem to be few correspondents with basic scientific literacy. Not even the ability to add up the numbers on an electricity bill or distinguish between the press release of a company planning to get wonderful results in 2025 vs today’s reality.

Anyway, entertaining as it is to shoot fish in a barrel, we will try to stay away from discussing newsotainment and stay with the scientific literature and IPCC assessments. Inevitably, we’ll stray a little.

I haven’t tried to do a comprehensive summary of the issues believed to impact humanity, but here are some:

  • sea level rise
  • heatwaves
  • droughts
  • floods
  • more powerful cyclones and storms
  • food production
  • ocean acidification
  • extinction of animal and plant species
  • more pests (added, thanks Tom, corrected thanks DeWitt)
  • disease (added, thanks Tom)

Possibly I’ve missed some.

Covering the subject is not easy but it’s an interesting field.

Read Full Post »

« Newer Posts