Carbon dioxide (CO₂) absorbs energy strongly in two regions of the infrared part of the electromagnetic spectrum. If there was no CO₂ in the Earth's atmosphere at all, the Earth's temperature would be around -20°C. CO₂ gets into the atmosphere in various ways, and at present there is more being added to the atmosphere than is being removed. The Earth’s temperature undergoes periodic variations – around AD 1000, there was what is known as a ‘Medieval Warm Period’, the warmest epoch in the past 2000 years, while more recently there was a ‘Little Ice Age’ centered around 1700. Some fire extinguishers create their carbon dioxide when activated by pressing the button, using a reaction between an acid and sodium hydrogen carbonate, though this type has fallen out of use. CO₂ is of course also formed when yeast uses the enzyme zymase to catalyse the fermentation of sugar into alcohol and CO₂.