What are the disputes within climate change science?

The basic science of climate change is straightforward and not in serious dispute. There is broad support for the following key propositions:

  • Global warming greater than 1.5 or 2 degrees Celsius risks irreversible damage to human and natural systems;

  • We are currently at high risk of warming of between 3 and 4 degrees Celsius before the end of the century;
  • Averting catastrophic climate change requires urgent, coordinated action.

The principal area of uncertainty relates to 'climate sensitivity' (i.e. the sensitivity of the climate response to increased concentrations of GHGs in the atmosphere) and related questions of feedback effects and carbon sinks. The IPCC reports are regarded by some as being conservative, partly because of their exclusion of feedback effects which are understood in principle but difficult to quantify.

What changes have occurred to date?

Climate change

Warming of the climate system is unequivocal, and since the 1950s,
many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, and sea level has risen. Each of the last three decades has been successively warmer than any preceding decade since 1850. Over the period 1880 to 2012, the planet, as a whole, has warmed by 0.85 degrees Celsius[3].

Currently GHG concentrations in the earth’s atmosphere are higher than at any point in at least the last 800,000 years[4]; and rising fast.

Ocean acidification

The pH of ocean surface water has decreased by 0.1, corresponding to a 26% increase in acidity[5]. This is probably more acidic than at any point in the last 20 million years[6].

What causes the changes?

Climate change

Greenhouse gases (GHGs) in the atmosphere absorb and retain energy from the sun. As their presence increases so the planet warms up (and vice versa).  GHGs sealed the fate of the other rocky planets in the solar system. Venus is too hot for life, having suffered a runaway greenhouse effect: its oceans boiled away and most of its carbon ended up in the planet’s atmosphere as a blanket of carbon dioxide. Its mean surface temperature is now 462 degrees Celsius. The reverse happened on Mars, which began life warm and wet with abundant water before its carbon dioxide became trapped in carbonate rocks[7]. Its mean surface temperature is now -63 degrees Celsius.

AR5 attributes the rise in concentrations of GHGs to ‘anthropogenic … emissions since the pre-industrial era.’[8]

Ocean acidification

When CO2 is absorbed in water, carbonic acid is formed, which then dissociates into hydrogen ions, increasing ocean acidity.

What is the projection for future climate change?

Climate change

The UN projects that global population will reach 9.2 billion by 2050; while the OECD predicts that the global GDP will quadruple by the same date[12]. Population and economic growth mean increasing energy consumption and will, without corrective action, drive levels of emissions ever higher.

AR5 concludes that, without additional efforts to reduce GHG emissions beyond those in place today, by 2100 the average global temperature will have increased by 3.7 to 4.8 degrees Celsius compared to the period 1850-1900. When natural climate uncertainty is factored in the range expands to 2.5 to 7.8 degrees Celsius[13]. The actual emissions scenario that will be realised is, of course, a product of our own decisions.

Ocean acidification

Ocean acidification is projected to rise by 100-150% by 2100[14].

What is the 'long-term goal'?

The long-term goal (LTG) is the upper limit of average global warming that Parties to the UN Framework Convention on Climate Change (UNFCCC), i.e. governments, have pledged not to exceed. Article 2(1)(a) of the Paris Agreement commits the Parties to:

Holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change.

How do we keep to the long-term goal?

There is a point at which the total volume of GHGs released into the atmosphere will cause the bath to overflow (i.e. send average temperatures above the long-term goal). Scientists can estimate that total (to varying degrees of probability) and policy-makers and courts can weigh the evidence, and apply the precautionary principle, in settling on an appropriate figure. This in effect is the 'carbon budget' we need to stick to to meet the long-term goal.

Current global warming is caused by the anthropogenic emission of greenhouse gases (GHGs), in particular CO2, into the atmosphere. These gases trap the sun's heat, causing a 'greenhouse effect'. As the concentration of GHGs in the atmosphere increases, so does average global temperature.

Although the science of climate change is complicated by issues of 'feedback effects' and 'carbon sinks', such effects can be modelled, and the total available 'carbon budget' consistent with a particular LTG can be estimated.

The Intergovernmental Panel on Climate Change (IPCC) is a scientific body established by the United Nations Environment Program (UNEP) and World Meteorological Organization (WMO) in 1988, under the auspices of the UN. It aims to acquire insight into all aspects of climate change, such as the risks, consequences and options for adaptation and mitigation. Its reports (which are generally considered to be conservative, and to underestimate the impact of feedback effects) provide the scientific basis for UNFCCC negotiations.

Its most recent report ('AR5') specifies the carbon budgets consistent with the 2 degree Celsius long-term goal to varying degrees of probability:

Multi-model results show that limiting total human-induced warming to less than 2 degrees C relative to the period 1861-1880 with a probability of  > 66% would require cumulative CO2 emissions from all anthropogenic sources since 1870 to remain below about 2900 GtCO2 … About 1900 GtCO2 had already been emitted by 2011[AR5, SPM, 2.1].

If, in other words we're to have a better than 66% chance of limiting average warming to less than 2 degrees Celsius, as of 2011, we could afford to emit a further 1000 GtCO2 into the atmosphere. Even then we would have a one in three chance of overshooting a goal which is already in excess of the 1.5 degree limit required to protect the more vulnerable regions of the world.

Have governments agreed a collective carbon budget?


Although a carbon budget is the obvious way to operationalise the long-term goal, and although the IPCC has specified a carbon budget consistent with the 2 degree goal, the Paris Agreement does not refer to a carbon budget. There, is, therefore, still no political agreement on the total amount of GHGs we can afford to emit in order to keep to the goal.

Will tackling emissions undermine economic growth?

Our economies remain heavily dependent on fossil fuels. A radical energy transition, like any process of necessary change, will be challenging and sometimes painful. This has led some to view economic growth and climate change as conflicting policy goals, or 'growth vs. climate.' In fact both common sense and economic research show the opposite is true: sustainable economic growth depends on tackling climate change.

The latest IPCC report, AR5, concludes that without additional measures to limit GHGs we are heading for warming of between 2.5 and 7.8 degrees by 2100. Even at the mid-point of this range (about 5 degrees) the consequences are likely to be so severe as to reverse economic growth and development gains. 

In 2005, Gordon Brown, then the UK Chancellor, commissioned Nicholas Stern, previously chief economist at the World Bank, to prepare a report on the economics of climate change. The report[16] concluded that unchecked climate change would entail a loss of consumption of between 5% and 20% by 2050; whereas the costs of tackling climate change would be only 1 %.

If the Stern report proved highly controversial, AR5 likewise concludes that the transition to clean energy will support continuing high levels of economic growth.

According to AR5, baselines scenarios for growth in global consumption over the century, ignoring climate change, range from 300% to 900%. Mitigation scenarios likely to limit warming to 2 degrees reduce this by only a small fraction – 3% to 11%. In other words global consumption by 2100 might have grown by 297% instead of a baseline projection of 300%; or 889% instead of a projection of 900%.[17]

AR5 states:

Very little is known about the economic cost of warming above 3 degrees C relative to the current temperature level.[18]

Economics is an uncertain science: if it's difficult to predict the political consequences of warming of 3 to 4 degrees Celsius, the economic impact will be harder still.

It should be obvious, however, that limiting warming to 2 degrees in combination with growth of 297% - 889% by 2100, would be a more favourable outcome in all possible ways (including the economic) than a world that had warmed by 3-4 degrees.

​In September 2015, Mark Carney, the Governor of the Bank of England, warned that climate change will lead to financial crises and falling living standards unless the world’s leading countries do more to ensure their companies come clean about their current and future carbon emissions. In a speech to the insurance market Lloyd’s of London, Carney said insurers were heavily exposed to climate change risks and that time was running out to deal with global warming.

Can the market support a transition to clean energy?

In economics an externality is a cost or benefit that affects a party that has not chosen to incur that cost or benefit. Unregulated markets with significant externalities lead to prices that do not reflect the full social cost or benefit of their transactions and are therefore inefficient.

It has been estimated that for each of the 35 billion tonnes of CO2 currently being emitted annually is causing $40 worth of damage to the planet, possibly much more[19]. The market might be corrected by ensuring such costs were reflected in the price of fossil fuels.

Unfortunately the opposite is happening: the IMF has recently calculated that global fossil fuel subsidies amount to $5.3 trillion a year (or $10 million a minute)[20].

With appropriate incentives and disincentives the market would instead be working to support the transition.

What is the relationship between 'mitigation' and 'adaptation'?

‘Mitigation’ (reducing the problem) refers to actions to prevent or limit climate change (eg by controlling emissions or preserving and developing carbon sinks). Adaptation (adapting to the consequences) is aimed at attempting to make nature, society and the economy less vulnerable to a changing climate (eg the construction of flood defences or the relocation of populations).

​There are two key points to make:

  • Applying the principle of 'the polluter pays', the polluter should bear the cost of necessary adaptation; and
  • Efficient adaptation depends on confidence on holding warming to the long-term goal. 'How high do the flood defences need to be?' The answer depends on how warm it's going to get.

Does geo-engineering present an alternative to cutting emissions?

Geo-engineering has been defined as ‘the deliberate large-scale manipulation of the planetary environment to counteract anthropogenic climate change’[21]. The prospect of increasing climate change demands serious consideration of all conceivable mitigations.

A leading proposal is to inject sulphur dioxide into the stratosphere where it could reflect the sun’s radiation, without causing air pollution.

There are, however, a number of concerns about such approaches. AR5 concludes:

Solar radiation management (SRM) involves large-scale methods that seek to reduce the amount of absorbed solar energy in the climate system. SRM is untested and is not included in any of the mitigation scenarios. If it were deployed, SRM would entail numerous uncertainties, side effects, risks and shortcomings and has particular governance and ethical implications. SRM would not reduce ocean acidification. If it were terminated there is high confidence that surface temperatures would rise very rapidly impacting ecosystems susceptible to rapid rates of change.[22]

More specifically scientific modelling suggests sulphur dioxide injections would disrupt the Asian and African monsoons.

[1] i.e. to the Vienna Convention for the Protection of the Ozone Layer


[3] See AR5, SPM, 1.1

[4] ibid. 1.2

[5] ibid.

[6] C. Pelejero et al, 2010, ‘Paleo-perspectives on ocean acidification’, Trends in Ecology and Evolution, 25,6, 332-4

[7] Such carbonates were conclusively identified by the Mars Spirit Rover. See R Morris et al., 2010: ‘Identification of Carbonate-Rich Outcrops on Mars by the Spirit Rover’, Science, 23 July 2010, vol 329, no 5990, pp. 421-4

[8] AR5, SPM1.2

Climate change in the Fertile Crescent and implications of the recent Syrian drought, PNAS vol.112, no. 11, Colin P. Kelley et al.,  3241–3246, doi:10.1073/pnas.1421533112




[13] AR, SPM, 3.4

[14] AR4. 10.4.2

[15] AR5, SPM 2.3

[16] See at

[17] See AR5, SR, 3.4

[18] AR5, SR, Box 3.1

[19] Wagner and Weitzman, Climate Shock – The Economic Consequences of a Hotter Planet, Ch 1, p. 23. For uncertainties regarding the social costs of carbon see AR5, WG3, 3.9.4,


[21] Royal Society, Geoengineering the Climate: Science, governance and uncertainty, London, 2009, p.1

[22] AR5, SPM 3.4

What are the disputes within climate change science?                                                                                         

What changes have occurred to date?                                          
What causes the changes?                                                                                                             
What is the projection for future change?                                                

What is the 'long-term climate goal'?  

How do we keep to the long-term goal?   

Have governments agreed a collective carbon budget?                                                              

Will tackling emissions undermine economic growth?                     
Can the market support a transition to clean energy?  
What is the relationship between ‘mitigation’ and ‘adaptation’? 

Does geo-engineering present an alternative to cutting emissions? ​​​​ ​​

Climate Change Q & A 

Loveday Abadida
Bayelsa State
November 2012