The kilogram (kg) is the base unit of mass in the International System of Units (SI). It is defined as the mass of the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder kept at the International Bureau of Weights and Measures (BIPM) in France.
Until May 20, 2019, the kilogram was defined by the physical prototype, but as part of the redefinition of the SI units, the kilogram was redefined in terms of fundamental constants. Since that date, the kilogram is defined by fixing the numerical value of the Planck constant (h) to be exactly 6.62607015 x 10^-34 joule-seconds.
This redefinition ensures that the value of the kilogram is now based on a constant of nature rather than relying on a physical object that could potentially change over time. It allows for a more precise and stable definition of the kilogram and other SI units.
The petagram (Pg) is a unit of mass in the metric system. It is equal to one quadrillion (10^15) grams. The petagram is commonly used in scientific and environmental contexts, particularly in discussions related to global carbon cycles, climate change, and large-scale ecological measurements.
To put it in perspective, one petagram is equivalent to 1,000 teragrams (Tg) or 1,000,000 gigagrams (Gg). It is a valuable unit for measuring and quantifying large quantities of materials, such as the mass of carbon stored in forests or the carbon emissions from human activities on a global scale. While not commonly used in everyday conversation, the petagram plays a crucial role in scientific research and discussions concerning the Earth's ecosystem and climate.