Climate change and the various attempts to limit it represent today one of the greatest challenges for global society. At the core of the scientific debate lies, above all, the question: “What is the contribution of human activities to global warming?” and, consequently, “How much of a real impact can ‘green’ policies have in containing this phenomenon?”
In this article, we will try to answer these questions. I have aimed to make it as comprehensive as possible, so as to provide all the information needed to understand the topic. We will examine all the factors — both human and environmental — that influence climate variation, learn who monitors climate trends, and explore the scientific positions that support anthropogenic (human-caused) effects on the climate as well as those that remain skeptical, providing evidence for each from the literature and scientific reports.
A portrait of the pre-industrial climate and its variations
Before the Industrial Era, Earth’s climate experienced natural fluctuations: from the milder phases of the so-called Roman Climate Optimum (around 200 BC–400 AD), through the “Medieval Warm Period” (around 950–1250 AD), to the cooling known as the Little Ice Age (around 1450–1850 AD). These periods, studied through various parameters – tree rings, polar ice cores, lake sediments, and corals – clearly show how the average global temperature varied by just a few tenths of a degree over the centuries.
The post-industrial climate
Following the Industrial Revolution, atmospheric levels of carbon dioxide and methane rose by about 50% and 164% respectively between 1750 and today – an increase unmatched in the past 14 million years. Before moving forward, let us take a closer look at greenhouse gases, and how these increases have been measured.
Greenhouse gases
CO₂ and CH₄ – carbon dioxide and methane – are the two major “greenhouse gases” present in Earth’s atmosphere.
Carbon dioxide is produced mainly through the combustion of coal, oil, and natural gas, through industrial processes such as steel and cement production, and through organic degradation (plant and animal respiration, decomposition). Deforestation also contributes to higher CO₂ levels because it reduces the ability of plants to absorb carbon. In terms of volume, CO₂ is by far the most abundant greenhouse gas emitted by human activities.
Methane is naturally emitted through anaerobic fermentation processes in swamps and rice fields, but also by livestock (ruminant digestion), landfills containing organic waste, and leaks in natural gas infrastructure. Although present in much lower concentrations compared to CO₂, methane is about 25 times more powerful than CO₂ in terms of greenhouse effect, and it too contributes significantly to the rise in global temperatures.
How do greenhouse gases work? Both gases trap part of the solar energy reflected by Earth, preventing it from dispersing into space: this is the “greenhouse effect” that regulates the planet’s climate. An excess of greenhouse gases—such as the one we have been witnessing since the beginning of the Industrial Era—leads to excessive warming of Earth’s surface, with impacts on ecosystems, hydrological cycles, and extreme weather events.
How do we know methane and carbon dioxide levels from 1750?
Scientists mainly rely on the study of air bubbles trapped in polar ice layers. When snow accumulates in Greenland or Antarctica, over time it compresses into solid ice. During this transformation, tiny air bubbles become sealed inside the layers. Each layer of ice – formed hundreds or thousands of years ago – contains a “sample” of the atmosphere of that period. By extracting ice cores several kilometers long, researchers can analyze these bubbles in the lab and measure with extreme precision the concentration of CO₂ and CH₄ up to 800,000 years back.
Assigning an exact age to each air bubble requires cross-checking methods. Scientists count visible annual layers (like tree rings), analyze oxygen and hydrogen isotopes to reconstruct seasons, and identify volcanic ash layers or radioactive signals (for example from nuclear tests in the 1950s) as “time markers.” On average, the trapped air is slightly younger than the surrounding ice (by a few decades), but multiple dating techniques allow for precise correction of this discrepancy.
Comparisons with other sources (tree rings, lake sediments, corals) provide independent checks and greater accuracy. For example, fossil wood samples show seasonal and annual growth variations linked to atmospheric CO₂, while lake sediments record pollen and isotopes that confirm the ice-core estimates. Thanks to these methods, we know that around 1750 atmospheric CO₂ was between 275 and 285 ppm (parts per million; about 50% lower than today’s ~420 ppm), while methane stood at 700–800 ppb (parts per billion; about 164% lower than today’s ~1900 ppb).
Who monitors the climate?
The IPCC, in its Sixth Assessment Report (AR6), states with “unequivocal certainty” that human activity has already caused warming of about 1.1 °C compared to pre-industrial levels (1850–1900), and that without drastic emission cuts, Earth is heading toward an average increase of 2.7–3.5 °C by 2100 – an increase unprecedented in the last 14 million years.
What is the IPCC? And how independent is it?
The IPCC (Intergovernmental Panel on Climate Change) is a scientific body established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). Its main role is to assess, transparently and rigorously, the available knowledge on climate change, its impacts, and possible strategies for mitigation and adaptation.
The IPCC’s work is organized into periodic “assessment reports,” drafted by hundreds of volunteer scientists from around the world. It also publishes “special reports” on specific topics (such as global warming, land and ocean systems, or soil carbon management) and “methodological reports” to support countries in calculating and reporting their emissions.
Each draft undergoes both intergovernmental and scientific peer review to ensure neutrality and accuracy. At the end of the process, the report is approved jointly by government representatives, authors, and reviewers. This dual procedure – scientific and intergovernmental – is what grants the IPCC its recognition as an independent global climate authority.
In plain terms, are there conflicts of interest within the IPCC?
The IPCC was long criticized for lacking, in its early years, any formal mechanism to manage potential conflicts of interest. In 2010, three scientists – including Roger Pielke Jr. – wrote in the prestigious journal Science that they found no policy for issuing corrections, no conflict-of-interest declaration, and no transparent system for selecting and vetting participants in its assessments.
Following these concerns – and the reputation crisis triggered by climate-related errors (such as the famous claim about Himalayan glaciers) – the IPCC commissioned an independent review by the InterAcademy Council (IAC) in 2010. In response to the IAC’s recommendations, in 2012 the IPCC adopted, in its official documents, “a rigorous conflict of interest policy with the aim of protecting the agency’s legitimacy, integrity, trust, and credibility.”
Today, this policy requires all authors and reviewers to:
- Disclose in advance any financial or professional ties (e.g., employment, consultancy, shareholding) with energy industries, lobbying groups, or institutions that might benefit from specific IPCC conclusions.
- Update declarations throughout their mandate if new interests arise.
- Be evaluated by chapter coordinators and by a dedicated committee to check the consistency of their declarations.
Even so, some NGOs and organizations (including 350.org and Greenpeace) have continued to highlight appointments of experts with strong ties to major CO₂ emitters, calling for stricter selection criteria and broader representation of perspectives, regions, and disciplines. However, a more conservative position holds that the dual review process (scientific and governmental), together with online draft transparency, already provides effective safeguards against undue influence.
Which non-human factors influence global warming?
Among the natural factors influencing global climate, the ENSO cycle (El Niño–Southern Oscillation) is perhaps the best known. El Niño is an abnormal warming of surface waters in the central-eastern Pacific Ocean along the equator, which alters atmospheric circulation and affects rainfall, temperatures, and winds across many regions of the world.
Normally, trade winds (blowing east to west) push warm Pacific waters toward Australia and Indonesia, causing heavy rainfall there. On South America’s west coast (Peru, Ecuador), colder waters rise from the depths (upwelling), bringing nutrients. During El Niño, these winds weaken or even reverse, allowing warm waters to accumulate in the central and eastern Pacific while upwelling decreases off South America’s coasts. This leads to abnormal ocean warming that can last 6 to 18 months.
Alongside ENSO, solar cycles modulate the energy Earth receives, affecting climate by a few tenths of a degree. These are natural variations in solar activity, with an average period of about eleven years, alternating between maxima and minima of sunspots and flares. Across each cycle, the average solar energy reaching the top of Earth’s atmosphere fluctuates by about 0.1%. In temperature terms, energy balance models estimate this causes global surface warming of about 0.02–0.05 °C, depending on the duration and strength of the cycle.
Volcanic eruptions of medium to large magnitude release sulfate aerosols into the upper atmosphere, reflecting solar radiation and causing cooling of a few tenths of a degree for 1–3 years.
On even longer time scales, orbital variations described by Milanković – eccentricity, tilt, and precession of Earth’s axis – modulate seasonal insolation with cycles of 23,000, 41,000, and 100,000 years, triggering the great glaciations and deglaciations of the Pleistocene.
To a smaller extent, suspended particles (aerosols) also add “variability” to the climate, like background noise. Aerosols can come from natural sources (desert dust, volcanic vapors) or human activities (burning coal and oil). They reflect part of the sunlight, reducing the amount of heat reaching Earth. However, this cooling effect is much weaker than the warming caused by greenhouse gases from combustion.
Clouds are even more complex: depending on their thickness and altitude, they can either reflect sunlight back into space (cooling) or trap heat emitted by Earth (warming). Small changes in cloud formation or thickness thus alter the balance between incoming sunlight and outgoing heat. Climate models still struggle to predict these effects precisely because they depend on microscopic details of water crystals and suspended droplets. Simply put: clouds and aerosols play an important role in “regulating” the climate day by day, and their influence is complex and not yet fully understood.
Equally important are longer-frequency ocean oscillations, such as the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO), which redistribute heat between oceans and atmosphere over decades or centuries.
Human impact on post-industrial global warming
Now that we have gathered all the necessary information, we can answer the ultimate question: given that global warming over the past 100 years is undeniable, how responsible is humanity, and could green policies really stop the steady rise in temperatures?
While the natural factors mentioned above play a minor role in influencing global temperatures, experts have confirmed beyond any doubt that humans are by far the main drivers of global warming.
How has this been proven? Climate scientists combine multiple lines of evidence. First, isotopic analysis of carbon in ice-core air bubbles shows that the rising share of CO₂ has a “fossil” signature – low in ¹⁴C – perfectly consistent with the burning of coal, oil, and natural gas, while natural processes alone cannot explain either the speed or the magnitude of this increase.
Second, a clear “climate fingerprint” emerges: computational models that include only natural drivers (ENSO cycle, solar variations, volcanic eruptions) cannot reproduce the global temperature trends of recent decades. Only by adding anthropogenic greenhouse gas emissions do models align with real-world measurements – in terms of tropospheric warming, stratospheric cooling, ice melt, and rising sea levels.
On the second question, regarding the contribution of green policies
There are no absolute “certainties,” but rather well-quantified scenarios and probabilities. According to the IPCC AR6 Synthesis Report, if the world managed to reduce emissions to net zero around 2050, there would be less than a 50% chance of exceeding a +1.5 °C increase by the end of the century. If, on the other hand, policies remain weak, the most realistic scenario would be average warming of about +2.7 °C, or even +3.6 to +4.4 °C in the worst cases. These projections take into account both the uncertainty of climate models and natural fluctuations in climate, such as El Niño or solar cycles, which may cause temperature swings of a few tenths of a degree over short periods, but whose weight is far smaller compared to warming caused by rising greenhouse gases.
Looking at only a narrow time window, a warming phase may seem consistent with Earth’s “natural” cycles, but the speed and scale of the temperature rise – over 1 °C in just a little more than a century – far exceed the millennial variations observed in paleoclimate proxies, and it is accompanied by unprecedented CO₂ and CH₄ concentrations. In other words, while natural oscillations in the short term cannot be excluded, the gap between a “climate without green policies” and a “climate with ambitious policies” remains so wide that decarbonization emerges as the only realistic way to brighten the climate future.
The skeptics’ arguments
Those skeptical about humanity’s role in global warming generally rely on three major arguments.
The first concerns the Sun. According to this view, changes in solar activity – for example in the length of solar cycles or in the intensity of radiation reaching Earth (Total Solar Irradiance) – would be closely tied to global temperature trends. In the 1990s and 2000s, studies such as those by Friis-Christensen and Svensmark suggested an apparent coincidence between solar cycles and temperatures, and between cosmic ray flux and cloud cover. The idea is that fewer cosmic rays mean fewer reflective clouds, therefore more heat reaching Earth’s surface.
The second argument relates to the cosmic ray theory: according to some scientists, such as Willie Soon and Sallie Baliunas, particles from space act as “seeds” for water droplets to condense on, forming clouds. If the flux of cosmic rays changes, cloud formation also changes, with consequences for the climate. These authors argue that this natural mechanism explains temperature data (including those from satellites and weather balloons) better than the hypothesis attributing warming to human-emitted greenhouse gases.
Finally, there is the appeal to natural climate cycles. Earth’s system shows large internal oscillations, such as El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO), Milanković astronomical cycles, and the natural variability of clouds and aerosols. From this perspective, these phenomena, which can last decades or millennia, would have enough amplitude and frequency to explain phases of warming or cooling without invoking human causes. In this view, the current temperature rise would be part of a natural cycle that climate models do not adequately represent.
What the science really says
However, subsequent analyses have shown that the correlations just discussed were based on incorrect data or too short time periods, that the variation in cosmic rays is 2–3 orders of magnitude too small to trigger significant cloud changes, and that when natural forcings (solar, volcanic, orbital) are included in models without anthropogenic greenhouse gases, the real temperature trend of the past 70 years cannot be reproduced.
Consequently, while acknowledging that nature has its rhythms, the scientific community now concludes unanimously that none of these natural mechanisms, alone or combined, can explain the magnitude and speed of the observed warming. Instead, the accumulation of human-produced greenhouse gases remains the explanation most consistent with all available evidence.
In summary, it is the convergence of isotopic evidence, climate fingerprints, and modeling – each independent and complementary – that validates the anthropogenic origin of the recent growth in greenhouse gases and therefore of Earth’s average temperature rise. At the same time, although no model can guarantee a future entirely free of uncertainty, the mitigation policies outlined by the IPCC offer the highest probability of keeping warming within still-manageable limits, minimizing the share of risk that would remain even with natural variability in the Earth system.
The key to not getting lost in a debate alternating between legitimate scientific reservations and established certainties lies in using all available knowledge. On the one hand, we should not ignore the complexity of the Earth system and its natural oscillators; on the other, we must recognize that the rapid increase in greenhouse gas concentrations has tipped the balance toward warming that nature alone would hardly have produced in such a short time.
“Green” policies – from energy efficiency to decarbonization, from soil restoration to the circular economy – are the most effective response to keep temperature rise within sustainable thresholds, such as those indicated by the IPCC. Every delay makes climate impacts more severe and the costs of adaptation and recovery much higher. Only an approach that combines mitigation and adaptation, grounded in robust and up-to-date data, will allow us to truly “govern” climate change – rather than suffer it passively – ensuring a safer future for the next generations.
Sources and material
Graphs
https://climate.nasa.gov/vital-signs/global-temperature/?intent=121
Overview of global warming
http://it.wikipedia.org/wiki/Riscaldamento_globale
http://en.wikipedia.org/wiki/Climate_change
http://climate.nasa.gov/
http://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature
Reports and official data
http://www.ipcc.ch/report/ar6/syr/
http://www.ipcc.ch/sr15/
http://public.wmo.int/en/our-mandate/climate/wmo-statement-state-of-global-climate
Natural factors influencing climate
http://en.wikipedia.org/wiki/El_Ni%C3%B1o%E2%80%93Southern_Oscillation
http://www.climate.gov/enso
http://it.wikipedia.org/wiki/Cicli_di_Milankovi%C4%87
http://science.nasa.gov/solar-physics/solar-activity/solar-variability-and-climate/
http://en.wikipedia.org/wiki/Atlantic_multidecadal_oscillation
http://en.wikipedia.org/wiki/Pacific_decadal_oscillation
Historical climate databases and charts
http://data.giss.nasa.gov/gistemp/ (NASA GISS Surface Temperature Analysis)
http://www.metoffice.gov.uk/hadobs/hadcrut5/ (Met Office – HadCRUT5 Global Temperature Data)
http://climatereanalyzer.org/ (University of Maine – Climate Reanalyzer, interactive charts and maps)
http://www.ncei.noaa.gov/access/monitoring/climate-at-a-glance/global/time-series (NOAA Climate at a Glance – global temperature time series)
http://cdiac.ess-dive.lbl.gov/ (Carbon Dioxide Information Analysis Center – historical CO₂ data)
http://scrippsco2.ucsd.edu/ (Scripps CO₂ Program – Mauna Loa time series)
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