Since the start of the Industrial Revolution in the late 1700s, man-made forcings, such as increased greenhouse gases and aerosols, ozone changes, and land-use, have played an increasingly important role in climate change.

However, looking at the last 10,000 years, the period after the last ice age, it was primarily natural climate forcings, such as changes in solar radiation and volcanic activity, that influenced the global climate. The contribution from natural forcings is both positive and negative, while the contribution from CO₂ is always positive and accumulates year after year.

Changes in the Earth's Position Relative to the Sun

The sun is the source of all life on Earth and a major driver of climate change. Total solar radiation at the top of the atmosphere depends not only on how much energy the sun emits at any given time, but also on the position and orientation of the Earth relative to the sun.

Variations in the Earth's position help explain long-term temperature changes on Earth, such as those during ice ages.

The position and orientation of the Earth relative to the sun can be described by the so-called orbital parameters:

1. The Earth's orbit is shaped like an ellipse, but the degree of the elliptical shape varies in cycles of 100,000 and 400,000 years.
2. The tilt of the Earth's axis relative to its orbit varies in cycles of approximately 41,000 years. The tilt of the Earth's axis is the reason we have seasons on Earth.
3. The Earth is like a spinning top rotating on its own axis over the course of a day. However, the Earth's axis will not always point toward the same spot. Instead, it will wobble slowly (precession). Due to this phenomenon, the date of perihelion (the point in the Earth's orbit where the Earth is closest to the sun) will change over time in cycles of approximately 19,000 and 24,000 years.

The current interglacial period has been characterized by a series of changes in orbital parameters. Approximately 6,000–7,000 years ago, a greater tilt of the Earth's axis contributed to increased solar radiation during summer in the Northern Hemisphere, while the tropics received less solar heat. At the same time, changes in the Earth's precession caused seasonal variations in the north to intensify.

These changes contributed to a sharp increase in summer temperatures in the Northern Hemisphere, which in turn can explain why large Norwegian glaciers such as Hardangerjøkulen and Folgefonna were gone at that time. In the tropics, it was generally colder while monsoon rains were stronger than today. This may help explain why the Sahara was a lush area during that period.

Variation in Solar Radiation

Total solar radiation varies over time and can be related to the well-known 11-year cycle in sunspot activity. Higher sunspot activity results in increased solar radiation to the Earth and vice versa. Direct observations of sunspots on the sun show that the latter half of the 1600s was a period of abnormally low sunspot activity. Since then, there has been a steady increase in sunspot activity until the present day. Direct measurements of solar radiation from satellites do not show any clear positive trend since 1978. This indicates that changes in solar radiation alone cannot explain the ongoing global warming.

Volcanoes and Climate

Volcanic eruptions are important for changes in weather and climate, especially during the first few years after the eruption. The most significant climate effect of volcanic eruptions is due to the emission of sulfur gases, which are rapidly converted into aerosols. The haze of aerosols reflects some of the incoming solar radiation, thereby leading to a cooling at the Earth's surface. Volcanoes also release water vapor and carbon dioxide, but due to the large amounts already existing in the atmosphere, even a large volcanic eruption will not lead to a noticeable change in global CO₂ concentrations. On very long timescales (thousands to millions of years), however, eruptions from many gigantic volcanoes can raise the concentration of CO₂ in the atmosphere enough to cause global warming.

The Bjerknes Centre has used the Bergen climate model, including volcanic eruptions and variations in solar radiation, to simulate the climate during the "Little Ice Age" (1400s–1850s). Natural forcings explain much of the variations in both reconstructed and simulated temperatures in the Northern Hemisphere for this period (see Figure 1). The coldest periods during the "Little Ice Age" tend to occur in periods with especially powerful eruptions, such as Kuwae in 1453 and Tambora in 1815, and in periods with a series of subsequent eruptions, such as during the 1600s.

Climate of the Last Hundred Years

Climate models can be run with natural forcings, man-made forcings, and both simultaneously. Figure 1 shows that it is only when researchers include both natural and man-made contributions that the model is able to simulate the long-term positive temperature trend observed over the last hundred years (the red curve in the lower part of the figure). According to the model, temperatures in the Northern Hemisphere should have fallen in the last few decades if it were not for the man-made emissions of greenhouse gases. This indicates that observed warming is primarily due to man-made emissions. 

On a short timescale, such as a ten-year period, natural variations can exceed man-made climate change and lead to some cold years, but in the long run, man-made warming will dominate the trend.