Climate Change: The Science

Climate Change and Global Warming

Take away Points:

  1. Carbon dioxide( CO2) emissions are the principal cause of Climate Change. Other greenhouse gases, mainly Methane also contribute.
  1. The levels of Carbon dioxide have increased from 270 ppm (parts per million) in the atmosphere to over 420 ppm in just 150 years. This rate of increase is unprecedented and the highest level ever over the last 3 million years.
  1. Since 1850 the average world temperature has risen by 1.1°C and continues to rise. It is scientifically indisputable that this is entirely due to man-made carbon emissions.
  1. 72% of all global Carbon dioxide emissions are due to fossil-fuel (Petrol, Diesel, Gas) energy production for electricity, transportation and industrial manufacturing. Consequently, replacing fossil-fuels by clean, renewable energy sources is the most mitigating action that can be taken to minimise climate change.
  1. To avoid unmanageable global catastrophes due to climate change such as massive drought and crop failures, extensive coastal erosion and flooding, more violent and frequent storms and devastating wildfires, global warming must be limited to 1.5°C and absolutely not beyond 2°C.
  1. Countries are having success in reducing GHG. Emissions of GHG in the European Union fell by 25% from 5.6 billion tonnes of CO2 equivalent in 1990 to 4.2 billion tonnes in 2018.

Questions that are answered and explained

How does carbon dioxide trap heat?

You’ve probably already read that carbon dioxide (CO2) and other greenhouse gases (GHG) act like a blanket or a cap, trapping some of the heat that Earth might have otherwise radiated out into space. That’s the simple answer. But how exactly do certain molecules trap heat? The answer there requires diving into physics and chemistry.

Simplified diagram showing how Earth transforms sunlight into infrared energy. Greenhouse gases like carbon dioxide and methane absorb the infrared energy, re-emitting some of it back toward Earth and some of it out into space.

When sunlight reaches Earth, the surface absorbs some of the light’s energy and reradiates it as infrared waves, which we feel as heat. (Hold your hand over a dark rock on a warm sunny day and you can feel this phenomenon for yourself.) These infrared waves travel up into the atmosphere and will escape back into space if unimpeded.

Oxygen and nitrogen don’t interfere with infrared waves in the atmosphere. That’s because molecules are picky about the range of wavelengths that they interact with, Smerdon explained. For example, oxygen and nitrogen absorb energy that has tightly packed wavelengths of around 200 nanometers or less, whereas infrared energy travels at wider and lazier wavelengths of 700 to 1,000,000 nanometers. Those ranges don’t overlap, so to oxygen and nitrogen, it’s as if the infrared waves don’t even exist; they let the waves (and heat) pass freely through the atmosphere.

A diagram showing the wavelengths of different types of energy. Energy from the Sun reaches Earth as mostly visible light. Earth reradiates that energy as infrared energy, which has a longer, slower wavelength. Whereas oxygen and nitrogen do not respond to infrared waves, greenhouse gases do.

With CO2 and other greenhouse gases, it’s different. Carbon dioxide, for example, absorbs energy at a variety of wavelengths between 2,000 and 15,000 nanometers — a range that overlaps with that of infrared energy. As CO2 soaks up this infrared energy, it vibrates and re-emits the infrared energy back in all directions. About half of that energy goes out into space, and about half of it returns to Earth as heat, contributing to the ‘Greenhouse effect.’

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A consequence of Carbon dioxide emissions is the warming of the global atmosphere- Global warming. With more heat and energy in the atmosphere, this has long-term affects on the climate. Around the world the climate is changing, we are in the era of Climate Change. Over thousand and  millions of years the Earth’s climate has naturally changed, but what is unique now is the rate of change. What would have occurred over thousands of years is now happening in decades. Weather patterns are more extreme and more frequent. Various regions around the world are experiencing unprecedented droughts or conversely floods. Storms are more violent, and the arctic ice is thinning. The 5 hottest years globally since records began in 1880 have occurred in the last 5 years.

Since 1900, the average global air temperature has increased by approximately 0.7°C, but since the start of the industrial revolution in 1850 the rise is even more dramatic at 1.1°C. This may seem small, but this is sufficient to expand the water in the oceans and melt  glaciers to the extent that sea-levels have risen on average by 21cm since 1900.

Various weather events in 2020 further i the effects of climate change

What other greenhouse gases are in the atmosphere ?

What other greenhouse gases are in the atmosphere?

Water vapour is part of the natural water/rainfall cycle on the Earth and excess is lost from the atmosphere as rain, although as the climate warms there is more water vapour in the air. Carbon dioxide levels in the atmosphere have increased dramatically from approximately 270 ppm(parts per million) before the industrial revolution of the 19th century to over 420 ppm today, and increasing. Carbon dioxide accounts for about 80% of all anthropogenic (Human generated) greenhouse emissions and is by far the major contributor to climate change, but human activity also generates other greenhouse gases such as Nitrous oxide and Methane. Greenhouse gases warm the atmosphere at different rates defined as their Greenhouse Warming Potential (GWP). For instance, a litre of Methane in the atmosphere is equivalent to 80 litres of CO2  but only persists in the atmosphere for 10 years, whereas the presence of CO2 can remain active for up to 1000 years. So even if the World stopped pumping CO2 into the atmosphere today, the legacy and effect of over 200 years of industrial activity polluting the atmosphere will remain for a very long time.

To simplify the analysis of all non-CO2 greenhouse gas emissions they are all converted into equivalent CO2 emissions CO2e.

In 2020, the World emitted 51 Billion Tonnes CO2e.

Why have carbon dioxide levels increased so rapidly?

The following graph displays the increase in average global CO2 levels over the period 1750 to 2019.

The increase in CO2 atmospheric levels correlates and has been scientifically proven to be caused by increased worldwide industrialisation and its associated human activities. These activities cover energy generation by fossil-fuels, transportation using internal combustion engines (petrol and diesel) and intensified farming which produces CO2 and enormous volumes of methane gas.

There have been natural fluctuations in CO2 levels over hundreds of thousands of years resulting in warmer and colder (Glacial Ice age) epochs in the Earth’s history, but the recent huge and rapid leap in CO2 levels to 420 ppm (2021) is unprecedented over the last 3 million years

How do we know CO2 levels historically and how are they measured today ?

One way to measure past temperatures is to study ice cores. Whenever snow falls, small bubbles filled with atmospheric gases get trapped within it. In some places, so much snow falls that the older layers become buried and compressed into ice, locking away air bubbles in ice sheets and glaciers. With extremely careful drilling, we can extract long ice cores from these features to study the thousands of layers ice representing separate snowfalls and their trapped air bubbles. In controlled laboratory environments, we can measure the chemical makeup of the air that has been trapped - how much oxygen, carbon dioxide, and nitrogen gas was present in the atmosphere at the time it was buried in the ice. From these measurements, we can calculate past temperatures using empirical data on how these gases hold heat in the modern atmosphere. The temperature record recovered from ice cores goes back hundreds of thousands of years from glaciers that have persisted on landmasses like Greenland and Antarctica.

Today, measurements are made around the world at approximately 100 observatories and locations. Satellite monitoring of surface temperatures using infra-red imaging is also an important source of data. The Mauna Loa Observatory of the National Oceanic and Atmospheric Administration at Hawaii, is the world's primary atmospheric carbon dioxide measurement centre since 1958. Its remote and high altitude in the Pacific ocean eliminates any local pollution interference in its measurements. All global measurements used by the World Meteorological Organization (WMO) vindicating the increase in atmospheric CO2 levels are accurate to within 0.2 ppm.

The Mauna Loa observatory takes daily air samples and produces The Keeling Curve, a graph of the accumulation of carbon dioxide in the Earth's atmosphere based on these measurements.

The Keeling Curve from 1960 to 2020.

The Keeling curve displays cyclic variations due to the seasonal growth of plants and forests. Most of this effect is from the Northern hemisphere where most of the land and vegetation is located. From a maximum in May, the level decreases during the Spring and Summer with new plant growth extracting CO2 from the atmosphere through photosynthesis. After reaching a minimum in September, the level rises again in the Autumn and Winter as plants and leaves die off and decay,   releasing CO2 back into the atmosphere. The seasonal variation is has no long term effect on the overall increasing trend.

Where do global carbon emissions originate?

72% of all carbon emissions are energy related. Industrial manufacturing processes (24%), road, rail, air and sea transportation(16%) and heating and air-conditioning in buildings (17%) account  for the majority of energy generated emissions. Industrial processes such as oil refining (5%), chemical production(2%) and Cement manufacturing(3%) also directly generate CO2 and other greenhouse gases. Agricultural activity is also a major CO2 contributor(18%), with livestock(meat production) being the main contributor in this sector (6%). Significant emissions of carbon and methane  are also liberated in land deforestation (2%) and soil ecosystem destruction in desertification and permafrost thawing (4%.

While carbon reductions in all sectors is essential to reach the target of a  carbon-neutral world by 2050, the elimination of fossil-fuel energy is the biggest and most important challenge for all countries. This is being achieved as more and more renewable energy sources such as solar and wind energy are being adopted and deployed.

Where are Ireland’s carbon emissions being generated ?

The biggest source of greenhouse gas emissions in Ireland is CO2 emissions from the burning of fossil fuels. All internal combustion engines (cars, lorries) and oil and gas heating use fossil fuel. In addition, 57% of  Ireland’s electricity generation (2020) still uses oil, gas or coal. Fortunately, the 43% from renewables, is a percentage that is increasing year by year and the target is to have 70% renewables for electricity by 2030. Combining total energy consumption (petrol/diesel, electricity from non-renewable sources) from all sectors, industry, agriculture, transport and residential, this accounted for 59% of Ireland’s greenhouse gas emissions in 2018.

In 2019, Ireland’s GHG emissions were 59.78 million tonnes CO2e. Emissions of CO2 accounted for 62.4 per cent of the total, with Methane and Nitrous Oxide contributing 24.6 per cent and 11.5 per cent, respectively.

Ireland is unusual compared to other EU countries because GHG emissions from agriculture is the sector with the  highest emissions. In 2018, agriculture was responsible for 33% of all greenhouse gas emissions.

Agriculture sector emissions arise from enteric fermentation (methane emissions arising from digestive process in livestock), manure management and nitrogen and urea application to soils. In addition, fuel combustion from agriculture/forestry/fishing is included. This sector contributed over 35.4% of Ireland’s total emissions in 2019 and is projected to rise to 40% by 2030.

Ireland had the third highest emissions of greenhouse gases per capita in the EU in 2018 at 12.6 tonnes of CO2e. This ranks Ireland  seventh worst out of 28 EU Member States in terms of its total greenhouse gas emissions at 89.0 relative to the base year of 2005=100.

The emission projections for 2020 by the EPA are illustrated below:

How does Ireland’s GHG emissions compare to other countries ?

GHG emissions in different countries can be compared in simple absolute terms by calculating a country’s Total Emissions from all fossil-fuel combustion.  While this identifies the world’s biggest CO2 polluters, it doesn’t take into account the average carbon footprint of individuals in each country. The Carbon footprint per capita (Total emissions/population), is a much more informative measure indicating how carbon -polluting a society is, in general. of a particular country. The following table lists total and per capita emissions for the biggest carbon emitters in the world for 2021.

China is the world’s largest CO2 emitter with 9.04 Billion tonnes CO2e/yr, followed by the U.S 5 Billion tonnes CO2e/yr and India 2.07 Billion tonnes/yr. However, in terms of per capita emissions (tonnes/yr per person) , the U.S is more than twice that of China (15.53 compared to 6.59) and almost ten times that of India (15.53 compared to 1.58). This reflects the enormous U.S consumer market that can afford to buy large energy consuming items such as cars and heating and air-conditioning units and is a general feature of the developed world.

Ireland’s total CO2 emissions(excluding methane) on a global scale are very small, 38 million tonnes/year out of a global total of 43 billion tonnes. While this represents only 0.1% of CO2 global emissions, it is still larger than the carbon footprint of the poorest 300 million humans in the world.

Taking into account Ireland’s methane emissions the CO2e per capita rises to 13.3 tonnes per year/person, the third highest in the E.U.

How can we predict the cause and effects of climate change?

Climatologists use highly complex computer models to simulate the behaviour of the atmosphere under various physical and chemical conditions. climate models are an extension of weather forecasting. But whereas weather models make predictions over specific areas and short timespans, climate models are broader and analyse much longer timespans. They predict how average conditions will change in a region over the coming decades.

Climate models take into account the physical and chemical interaction of the Atmosphere, Cryosphere (Ice in the Arctic, Antarctica, glaciers, lakes and permafrost), Hydrosphere(Liquid water in oceans, rivers and lakes) and Lithosphere (Earth’s crust and rocks). Carbon emissions and natural volcanic activity which cause the Earth’s climate to warm are termed  Positive Forcings whereas some aerosol emissions (volcanic and industrial sulphur gases) and snow cover reflect solar radiation and cool the atmosphere and are termed Negative Forcings. Unfortunately, positive forcings are very dominant due to human activity.

Scientists use three common types of climate mathematical models which in increasing order of complexity are termed Energy Balance , Intermediate Complexity, and General Circulation models. The accuracy of these models’ can be verified by comparing their simulated predictions using historic data against the  climate events and trends of the historic period. The test process is called “Hind-casting”.

All mathematical models  visualise the atmosphere as  a 3-dimensional grid of cells that interact with their neighbours.

The exchange of energy, moisture and diffusion of GHG between cells are described by complex, scientific physical and chemical equations. These equations simulate the behaviour of the atmosphere, in a series of small incremental timesteps. The accuracy and predictive capability of a model (10-50 Kms to 100s Kms) depends on the granularity of the cell size and the size of the time steps(hours, days, weeks). The smaller the cell resolution and the smaller the time step, the finer the detail. However, greater accuracy increases the numbers of cells and time steps and the amount of computation and necessitates the use of some of the world’s most advanced supercomputers.

Regardless of which models are used, all predict and concur that increased carbon emissions will increase global warming with consequential glacier and ice loss, rising ocean levels, more frequent and violent storms and a greater incidence of droughts and floods worldwide.

Furthermore, all the models can only accurately predict the current climatic conditions that we are experiencing, by incorporating the man-made carbon emissions that have occurred since the 1850’s. Thousands of climate simulation runs prove irrefutably, that climate change is a direct consequence of human induced carbon emissions. The predictions also correlate and are confirmed with a mass of scientific evidence gathered from research stations, centres and laboratories around the world involved in the investigation of climatic effects on oceans, the environment, eco-systems and meteorological data.

The UN’s IPCC (Intergovernmental Panel on Climate Change) 6th Assessment Report 9th August, 2021, states that it is indisputable that humans are responsible for the recent climate change. The 3,500 page report is the conclusion of 234 leading climatologists and scientists from over 90 countries who reviewed over 14,000 research papers and articles. The headlines of the report state:

  1. It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred.
  1. The scale of recent changes across the climate system as a whole and the present state of many aspects of the climate system are unprecedented over many centuries to many thousands of years.
  1. Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since the Fifth Assessment Report.

The conclusions of the 6th IPCC Assessment report, reiterate and urge immediate action on the agreed outcomes of the 2015 Paris Climate agreement agreed by 196 countries:

To hold 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.

It is imperative that we recognise the reasons for this limit and realise that the 1.5°C   is not an arbitrary number subject to political debate and compromise, it is science, Nature’s non-negotiable mandate.

What are the predictions and consequences for the Earth with climate change ?

The climate is getting warmer, and even if all emissions were to stop today, it would not halt this trend. Carbon dioxide levels are so high that its legacy will continue to affect the climate for hundreds of years. Technologies to extract carbon dioxide from the air are being researched, but none at the moment are economically practical  or feasible solutions that could sequester carbon dioxide at the level of hundreds of millions of tons necessary to have an impact. Trees naturally extract carbon dioxide from the air during photosynthesis and the carbon is stored its wood and the soil. A mature 25 year old deciduous tree like an Oak or a Lime tree growing in Europe will absorb between 20-25 Kgs per year. Two-thirds of all the emissions from human activities that remain in the atmosphere today could be removed by 1.2 trillion newly planted trees, but this would require a land area the size of the U.S and China combined.

While the world’s forests act as a carbon sink absorbing 7.6 billion tonnes per year, the concentration of atmospheric carbon dioxide over the next decades is going to be largely determined  by how human society responds in its use and generation of energy. The degree by which emission reductions are achieved through greater energy efficiencies, adoption of more renewable sources, and change in lifestyles will be pivotal in the future trajectory of global warming.

If we are to keep global warming to within 1.5°C above pre-industrial times by 2100 then we must reduce carbon emissions to 50% of current levels by 2030, reach zero emissions by 2050 and approximately a 10% Negative (implying a 10% sequestration/absorption

Carbon reduction per year by 2100.  A 2% target would slightly relax these conditions. Current environmental policies and actions by governments worldwide will see global warming rise to between 2.7°C and 3.1°C by 2100. Non-binding pledges and targets that governments have made, would limit warming to about 2.4°C above pre-industrial levels. An “optimistic” targets scenario analysing the effect of net zero emissions targets of 131 countries that are adopted or under discussion estimates a temperature rise of 2.2°C. These various global temperature scenarios are shown in the following diagram.

The severity of the negative impacts  due to global warming on cities, food supply, biodiversity and the living environment for society are outlined below. Each half degree increase in global temperature delivers enormous incremental damage and devastation as ocean levels rise and weather extremes become more frequent.

90% of the excess heat generated by climate change is absorbed by  the oceans. The rising sea temperatures cause glaciers to melt and the ocean waters to expand and global sea-levels to rise.

Since 1900 sea-levels have risen 13-20 cms, more than the previous 2000 years. If just one third of Antarctica’s ice melts, the sea-level will rise 20 m. With increasing CO2 levels, the oceans are becoming more acidic. This, and rising temperatures is destroying the delicate Coral reef ecosystem. Occupying less than 1% of the ocean floor, coral reefs are home to more than 25% of  all marine species.

The human, environmental and societal consequences of exceeding the 1.5°C temperature ceiling are immense and irreversible. Virtually all low-level island states in the Indian ocean e.g. Maldives and the Pacific e.g. Tuvalu will be submerged. Over 40% of the world’s population live within 60 miles of the coast so major cities will be frequently flooded or even uninhabitable including Dhaka(20 million), Shanghai (17.5 million), Osaka(5.2 million), Miami(2.7 million), Rio de Janeiro(1.8 million), Alexandria(2.5 million) and  The Hague (2.5 million).

CO2 is acidifying the oceans as well as raising its temperature. This will kill coral reefs and deplete fish stocks. Higher temperatures will cause harvests and eco-systems to fail, and prevent manual work outdoors in fields where the temperature will exceed 45°C through the risk of heat stroke. As vast populations migrate for survival, inevitably social-political stresses and conflicts will emerge, unparalleled in history, that will exacerbate the situation.

The Arctic permafrost contains double the amount of CO2 and Methane as the entire atmosphere - almost 1.6 trillion tons. This is thawing and liberating its GHG reservoir.

Every kilogram of CO2 we can reduce, keeps the world closer to the lower target of 1.5°C.

However, this Doomsday scenario is avoidable, if action and responsibility at a political, social and individual level is taken to ultimately eliminate all GHG emissions and invest in new renewable and energy efficient technologies and adopt healthier and greener lifestyles. There is still time and  solutions and technology exist to achieve a sustainable, carbon-neutral world. Furthermore, every year we delay or procrastinate and don’t take the necessary steps to stop climate change we will add another 500 billion US dollars to the cost of climate inaction by 2030.

In fact, the economic and social opportunities of a sustainable world if embraced will provide the jobs and industries of the future, that will not just displace but greatly exceed those employed in fossil-fuel and other environmentally destructive industries. This is already happening. In the U.S 44,000 workers are employed in coal-mining which is dwarfed by the U.S renewable energy sector which employs 3.2 million and expanding.

Is climate change affecting Ireland’s weather ?

The Status of Ireland’s Climate report, August 2021, by University College Cork, Environmental Protection Agency (EPA), Met Éireann and the Marine Institute. Its conclusions state categorically that our weather is shifting to more extremes due to climate change.  Some of the key findings are:

  • Compared to pre-industrial levels, GHG are up 50 per cent, methane has soared by almost 170 per cent and nitrous oxide concentrations are around 20 per cent higher.
  • Ireland’s rainfall was 6 per cent higher in the 30-year period between 1989 and 2018, compared to the previous three decades. The decade between 2006 and 2015 was the wettest on record.
  • Average air temperatures also continue to rise, up by 0.9 degrees over the last 120 years. A rise in temperatures has been recorded in all seasons throughout the year.
  • The length of warm spells has also increased slightly over the past 60 years.
  • Sea levels have been rising by about 2-3mm every year since the early 1990s.
  • In the west, between 1972 and 2017 increased rainfall increased the  incidence of river flooding, while in the  east and south, droughts have been more prevalent in the Summer.

Can CO2 be extracted from the atmosphere ?

CO2 remains in the atmosphere for hundreds of years, whereas other GHG such as Methane last several decades which is still long enough to contribute to damaging climate change. So, even if all GHG emissions stopped today, their presence will have a lasting effect for a very long time. Scientists are exploring the possibility of Sequestering CO2 from the atmosphere, the process where CO2 is captured from air and then solidified or liquified and stored in underground caverns, or in the cold depths of the oceans. At the moment most attempts are too uneconomic to be practical, considering that millions of tonnes of CO2 would have to be captured. However, enhanced rock weathering in soils has substantial technical and economic potential as a global strategy for removing atmospheric CO2. When crushed basalt or other silicate material is added to soil, it slowly dissolves and reacts with CO2 to form carbonates. (

For the moment, the most cost effective and practical method of carbon capture are trees. Depending on age, size and geographical location trees can absorb between 20-25Kgs CO2 per tree/year. They are natural, prevent soil erosion, increase biodiversity, serve as a food source with their fruits and nuts, and provide carbon-neutral building building materials…and they are just beautiful to look at. We just need to preserve and grow more of them.

Trees are just Brilliant: They are natural carbon dioxide extractors

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