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Climate Models Are Developed With The Purpose of Experiments

While scrambling for answers to crucial queries, climate models are basically developed only to be relied upon. A diverse range of questions such as the reason behind the change in earth’s climate, the possible fallout of such change, changes to be occurred in future if greenhouse gas emissions remain unchanged, like now.

Models are immensely useful in determining observed warming in the past and the crucial role natural factors play as compared to man-made factors.

As a matter of factly, in an attempt to simulate climate of past times, present eras and of future, a wide array of experiments are performed by scientists. Besides, specific tests are also devised to evaluate the functioning of particular components fixed in climate models. Experiments are run to assess as to what would turn out if, CO2 is made to be quadrupled or if cooling climate needs geo-engineering approaches.

Model ensemble is created when lot many groups resort to similar kind of experiments on their climate models and with such model ensembles, researchers are empowered to look into the details harbouring climate models and the uncertain times lying in projections of the future.

Presenting a handful of experiments that modellers generally perform which form a part of Coupled Model Inter-comparison Projects (CMIPs) are as follows:

Historical Runs:

Climate models cover a wide spread of historical period ranging from 1850 through somewhere near present and supreme estimates of factors are taken into consideration which affect climate such as CO2, CH4, and N2O concentrations, output in sun’s changes, aerosols emanating from volcanic eruptions and other changes emanating from land use.

To record actual temperatures and rainfall, such historical runs not perfect devices, but such emerge from model physics, meaning that scientists are empowered to bring out a comparison between model predictions (known as hindcasts) based on climate of the past with climate readings recorded thereby. Now, if premium hindcast past climate variables are managed, which may include surface temperature, scientists are empowered to carrying out model forecasts pertaining to future.

Dear readers, this is worth mentioning that such historical runs should never be the criteria for any pre-recorded temperatures or rainfall but such runs are output from model physics itself. This underlines that scientists are empowered to bring out a comparison between model predictions (“hindcasts”) of earlier climate and other pre-stored climatic observations. Now, it is indeed encouraging to scientists in forecasting models of the future, if climate models manage to hindcast past climatic variables in successful way.


Besides, historical runs are handy in ascertaining the big role of human contribution in bringing about a climate change, known as attribution. Citing an instance, we present a graph here showing comparison between a couple of model variants and observed climate where natural forcing are in blue and model runs are in pink suggestive of man-made and natural forcing.

Human and Natural Impact On Climate

Graph here

Now, recording changes in sun’s energy and volcanoes are covered under natural only runs but assumption remains that greenhouse gases and other factors remain constant at pre-industrial levels. Now, with human only runs, natural factors don’t undergo any change but include the impact of manly activities such as greenhouse gas concentrations which rise in the atmosphere.

Now, by going for a comparison between these two, (with an all factor run) scientists can easily determine the singled-out contributions to observed climate changes arising out of such factors which can be man-made and natural too. Clearly, this enables them to unravel the proportion of modern day climate change which can be the result of man-made factors.

Warming Scenarios Of Future:

When the IPCC’s fifth assessment report was tabled, the focus was upon 4 warming possibilities of the future, which were termed as the Representative Concentration Pathway (RCP) scenarios. The emphasis was upon gauging the attainable climate change from now through 2100 and beyond.

Scores of factors would propel future emissions and such are whimsical in nature (namely- population and economic growth) and thus, such scenarios have to pass through a wide variety of future types —like business-as-usual world where concerned efforts are not so much cared about (RCP 6.0 and RCP 8.5) then we have a world, which is replete with rigorous mitigation efforts that are put and which check warming by more than 2C (RCP 2.6).

Such RCP scenarios indicate radiative forcing which vary in quantity and such forcing are utilized by models while examining earth’s system and how would it undergo a change against different pathways. Now, the CMIp6 exercise which is due, associated with IPCC 6th assessment report, will add another 4 new RCP scenarios to fill the vacancy revolving around 4 already in use and which would also include a typical scenario that satisfies the 1.5 C temperature limit.

Control Runs:

Such have proved to be immensely useful when we try to examine the way natural variability is expressed in models, which would be devoid of other changes. Control runs are also utilized where we try to diagnose “model drift” and where fake changes with lengthy spells take place in the models and which are distinct from natural variability or any changes occurring to external forcing.

Now, if a model is found to be drifting, there will emerge changes other than yearly ones and which take place in decades variability, while the factors affecting the climate remain static such as greenhouse gas concentrations.

Under model control runs, models is treated for a period falling before major industrialization as it enhanced the greenhouse gases dramatically. Following this, model is allowed to cover hundreds or thousands of years but without affecting greenhouse gases, solar activity or any such factors which would have affected the climate externally.

Clearly, such models are distinct from natural-only run because both man-made and natural factors are not involved at all.

AMIP Runs: Stands for Atmospheric Model Inter-comparison Project:

In climate models, land, ocean and atmosphere are well- represented. During such runs, every entity is switched-off other than atmosphere and for land and ocean, fixed values are used on the basis of observation.

As an example, during AMIP runs, as an input, observed sea surface temperature is used in the model which makes land surface temperature and the temperature recorded in multiple layers responsive. Generally, it is seen that climate models are designed with internal variability and climate cycles which appear in oceans for a short term like El Nino and La Nina scenarios—such take place at distinct time intervals. Now, with AMIP runs, modellers are empowered to bring about similarity between ocean temperatures and observations, with the target of letting internal variability in the models to occur at simultaneously just like in the observations, changes are recorded overtime in such runs can be easily compared.

Abrupt 4XCO2 Runs:

There are projects aimed at climate models comparison, such as CMIP5, wherein the requirement is that all models pass through “diagnostic” scenarios in order to validate performance spanning various criteria.

Among such tests, there is an “abrupt” surge in CO2 which is from pre-industrial times and is made to rise upto 4 times higher, i.e. 280 parts per million (ppm) to 1120ppm, combining such factors that wield an impact on climate constant. As a result, scientists are empowered to scan through the earth’s temperature alacrity in responding to changes taking place in CO2 in their model in comparison to others.

1% CO2 Runs:

This is yet another diagnostic test which impacts the CO2 emission increase dating back to pre-industrial levels, but with the addition of 1% per annum, till the time CO2 eventually quadruples and touches the 1120 ppm point. Such scenarios also retain the remaining factors as unchanged which would otherwise affect the climate.

As a result, modellers are allowed to single-out the effects of CO2 which rises slowly in everything else and leads to more complex scenarios later ahead, such as changes occurring in aerosols and in other greenhouse gases, like methane.

Palaeo-climate Runs: Under this category of runs, models are operationalized on past climates, i.e. Palaeoclimate. Models have been developed to run covering a wide range of periods, such as:

  • the past 1000 years,
  • the Holocene, which covered the time, past 1200 years,
  • Last glacial which was the time that was 21,000 years ago at length,
  • For ice age, the inter-glacial that was around 127,000 years ago,
  • The mid-pliocene warm period that was recorded 3.2 million years ago, and
  • The troubled period of rapid warming which was known as the Paleocene-Ecocene thermal which fell about 55 million years back.

Needless to say, such models make use of some chic calculations which scientists preserve for factors that impact Earth’s earlier climate and which also includes solar output and volcanic activity and then certain changes that occur in longer term in earth’s orbit and which is also shaken by the placement of continents.

Arguably, with such palaeoclimate model runs, researchers are immensely empowered to figure-out the earlier swings in the Earth’s climate, like those occurring in ice ages and what change was reflected by sea level and other such factors when warming and cooling periods ranged. With such past climate insights, we get an easy glimpse into the future and could easily ascertain the continuity of warming spell.

Specialized Model Tests:

Clearly, many research groups are hyper-active around the world in carrying out numerous experiments which is the part of overall CMIP6. Such include having a look at the reaction of aerosols in models, formation of clouds, ice-sheets reaction to warming, monsoon changes, rising sea levels, changes occurring in land use, oceans and the volcano effect.

Finally scientists also contemplate a geo-engineering model inter-comparison project, where the target will be to record the response of models to the release of sulphide gases into the stratosphere in order to cool down the climate, among other such principle disruptions.



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