Weather and Climate

Weather and Climate

Weather- is the state and atmosphere at a specific time and place

  • Key elements are temperature, precipitation, humidity, wind, and atmospheric pressure

  • Experienced day to day

Climate- can be related to the statistical probability that any day during the year will be similar to the same day in previous or following years

  • The average of weather over time, typically a minimum of 30 years

  • Key factors are temperature, precipitation patterns, and seasonal variations

Climate Variability- variations in climate patterns, beyond individual weather events, that occur within smaller timeframes, such as a month, a season, or a year.

  • Generally attributed to natural causes and occurs regularly

  • It can be caused by changes in ocean conditions far away, which can affect climate all over the world (e.g. El Niño)

Difference between Weather and Climate

  • Weather

    • Short term (hours to days)

    • Localized

    • Highly variable

  • Climate

    • Long-term (years to centuries)

    • Regional or global

    • More stable but can change over time

  • Analogy “weather is your mood, climate is your personality”

Climate Change- long term changes in temperature, precipitation, wind patterns, and other aspects of Earth’s climate system

  • Climate change refers to long-term shifts in temperatures and weather patterns

  • Such shifts can be natural, due to changes in the sun’s activity or large volcanic eruptions

  • Since the 1800s, human activities have been the main driver of climate change, primarily due to the burning of fossil fuels like coal, oil, and gas.

  • Burning fossil fuels generates greenhouse gas emissions that act like a blanket wrapped around the Earth, trapping the sun’s heat and raising temperatures,

  • Change of climate that is attributed directly or indirectly to human activity, altering the composition of the global atmosphere.

Is the climate changing?

  • Past century, global temperatures have risen at a pace that feels gradual in a human lifetime, yet it’s fast enough to raise serious concerns among climate scientists

  • Climate varies naturally- within limits

  • While individual extreme weather events don’t prove climate change, scientists have found a statistically significant rise in record heat waves and a decline in record cold spells over recent decades

How stable is the Climate?

  • Scientists reconstructed historical temperatures using tree rings, ice cores, sediments, and other proxy data.

  • Global temperatures have remained relatively stable since ice sheets retreated ~9,000 years ago

  • After a peak around 8,000 years ago, a slow cooling trend persisted until the late 19th century

  • Regional climate shifts (e.g in Africa, Europe and North America) impacted human settlement patterns

  • Past 5,000 years, civilizations thrived under stable climate conditions ideal for agriculture and trade

  • Climate records are being set across the world as heat waves, droughts, floods, snowstorms, tornadoes, and hurricanes impact millions of people

  • Levels of CO2 in the atmosphere have risen over 40% since preindustrial times; higher than at any other time during the previous 800,000 years

Historical Climate change

  • 10th-12th centuries: a warming period in northern hemisphere brought prosperity, population growth, and viking expansion

  • 17th-18th centuries: Climate cooling led to war, famine, and disease— a period called the Little Ice Age(LIA)

Climate reconstructions confirm two major events:

  • Medieval Climate Anomaly (MCA) ~1,000 years ago

  • Little Ice Age (LIA) ~400 years ago

These patterns reflect natural climate variability, not human-caused change

Recent climate change is well documented through historic records and scientific evidence

Hockey Stick Graph

  • In 1998, Michael Mann et. al, published a pivotal study reconstructing climate over the past 1,000 years.

  • The result: a graph shaped like a hockey stick-

    • Shaft- Gradual cooling over centuries

    • Blade- Rapid 20th century warming

  • Initially controversial, the graph suggested the MCA and LIA were regional or transient

  • Since then, multiple studies have confirmed the original findings with independent data and methods.

  • Red line: Recent global temperature rise is unprecedented in the historical record and linked to human activity

Recent Climate Change

  • The human influence on climate became evident by the mid-20th century, as industrial activity surged post- WWII

  • Scientists observed rising levels of atmospheric Carbon Dioxide (CO2), a known greenhouse gas that traps earth’s outgoing heat

  • Carbon isotope analysis confirmed the CO2 increase was due to the burning of fossil fuels

  • This excess carbon was found to be accumulating in both the atmosphere and the oceans

Temperature Records

  • The instrumental temperature record is relatively short in the context of climate history

  • The oldest continuous record comes from Central England, dating back to 1659

  • Reliable regional records began in the 1850s, driven by modern trade and global outreach, particularly under the British Empire

  • Truly global temperature data only became possible in the 1970s with the advent of weather satellites

  • Despite its limits, this record has been critical in detecting trends in modern climate change

Recent Observations on Land

  • In 2020, Northern Hemisphere land temperatures reached a record high of +1.74 degrees C (+3.13 degrees F) above the 1901-2000 average

  • In 2021, temperatures slightly decreased but remained high at +1.54 degrees C (+2.77 degrees F)

  • These Figures, based on the climate baseline, reflect a rapid warming trend

Why this matters: 

  • A seemingly small global increase translates into much larger regional impacts

  • A further rise of just 1 degree C (1.8 degree F) could make global temperatures the highest in 1 million years

  • Climate models project up to +4 degrees C (+7.2 degrees F) warming  by 2100, with devastating consequences for ecosystems and societies

Recent Observation in the Oceans

  • The world’s oceans have absorbed up to 90% of the excess heat from recent global warming

  • A clear warming trend is observed in the upper 700 meters (~3,000 feet) of the ocean

    • In 2020, the global average ocean temperature was +0.76 degrees C (1.37 degrees F) above the 20th-century average globally

    • Up to +0.99 degrees C (1.78 degrees F) warmer in the northern hemisphere

  • While these numbers seem small, the oceans hold immense thermal energy

The Role of Greenhouse Gases:

•Greenhouse gases trap heat radiated from Earth’s

surface, slowing its escape to space.

•Over 90% of global warming between 1971 and 2023

has been absorbed by the oceans.

•This has caused a significant increase in upper ocean

heat content.

Ocean vs. Air Temperature:

  • Public discourse often focuses on air temperature, ignoring the ocean's dominant role in Earth's heat balance.

  • Oceans cover 70% of Earth’s surface, storing vast amounts of heat.

  • However, warming is not uniform: some ocean regions show local cooling.

Critical Insight:

  • If the heat stored in the oceans were instantly transferred to the atmosphere, global air temperatures could rise by over 22 degrees C (40 degrees F)

A century of Warming

  • Global atmospheric temperatures (1800-present) show year-to-year variation due to:

    • Solar cycles

    • Volcanic eruptions

    • Atmospheric fluctuations 

    • Oceanic events (e.g. El niño, La niña)

The beach analogy:

  • Observing climate trends is like watching the tide at the beach

    • Waves = short-term natural variability 

    • Tide = long-term climate trend

  • Just as watching a few waves won’t reveal the direction of the tide, short-term climate data may obscure long-term warming

  • Key takeaway; Only through long-term, systematic observation does the underlying warming trend become evident, rising above natural fluctuations

Emerging from the LIA

  • ( 1880-1909:  a period of climate stability)

    • Solar output remained relatively stable

    • High volcanic activity introduced aerosols that reflected sunlight and caused global cooling

  • (1910-1945: Natural warming phase)

    • Increased sunspot activity led to a rise in solar radiation

    • Decline in volcanic eruptions reduced atmospheric aerosols, allowing more solar energy to reach Earth

    • Resulted in a noticeable warming trend, marking the end of the Little Ice Age

  • Attribute of Warming:

    • While some human influence may have contributed, climate models show this warming can be largely explained by natural factors

Cooling off Again

  • 1946-1975: a pause in warming; Warming flattened(late 1940s-1950s)

  • The early 20th-century warming trend stalled or reversed briefly

  • Caused by a combination of natural and human factors

    • Solar cycles (decreased solar radiation)

    • Volcanic activity (temporary cooling effect)

    • Industrial aerosols from post-WWII manufacturing, which reflected sunlight

  • Public vs Scientific view

    • Media speculated about a potential new ice age

    • But scientists at the time- and especially now- recognized that long-term warming was still underway

  • Short-term variability in temperature does not negate the long-term trend of global warming

A Human Element Emerges

  • Post-1975 A Clear Signal of Human-Caused Warming

  • Global warming accelerates after 1975

    • A strong warming trend resumed

    • For the first time, climate models could not reproduce observed temperature changes using only natural factors

  • Key scientific finding:

    • Models required the inclusion of greenhouse gas emissions to match observed warming

    • This marked the first robust evidence that fossil fuel combustion was significantly impacting Earth’s climate

    • Only models that include human influence successfully replicate observed temperature trends

    • Confirms that natural factors alone cannot explain the post-1975 global warming

Uneven Global Warming Patterns

  • Earth is Warming, but not evenly

    • warming confirmed by ground stations, weather balloons, and satellites

    • Compared to the 1901-2000 average:

      • Land warmed by +1.59 degrees C (2.86 F) in 2020

      • Oceans warmed by +.76 degrees C (1.37 F)

  • Regional Differences:

    • Arctic: Warming twice the global average rate

    • Antarctica: some areas are cooling, but extremes are still rising

      • Esperanza Base (2020) Record summer high 18.3 degrees C (64.9 F)

  • Regional variability matters-some areas warm drastically faster than others, impacting ecosystems and communities unevenly

Has warming stopped?

  • Observed trends (1975-2023)

    • Rapid Warming: 1975-2005 saw sharp global surface temperature increases

    • Apparent slowdown: 2005-2012 warming rate temporarily slowed, despite being the warmest decade on record at that time

    • Renewed acceleration: temperatures rose again significantly between 2016 and 2023

  • Key clarification

    • Short-term variations do not contradict long-term warming trends

    • Climate skeptics misinterpreted the 2005-2012 slowdown as an end to global warming 

  • Scientific consensus:

    • Basic physics and climate models project:

      • +1.8 degrees C(3.2F) warming at minimum by 2100

      • Up to +5.7 degrees C(10.2F) if emissions remain unchecked due to positive climate feedbacks

  • Global warming has not stopped— it’s accelerating.

Lessons from the Deep Past

  • The Cretaceous Period (~145-66 million years ago)

    • A natural “hothouse” climate: Global temperatures were significantly warmer than today

    • Fossil, isotope, and geochemical evidence support

      • Ice-free poles- Antarctica covered by lush forests

      • High sea levels and warm oceans

    • This was a time when:

      • Dinosaurs thrived in diverse ecosystems

      • Flowering plants and early grasses began to evolve

    •  The geological record confirms climate change as a natural earth process

    • But current warming is unprecedented in speed and clearly linked to human activity, unlike past natural changes

From Hothouse to Icehouse: Earth Climate Transitions

  • The Icehouse world- our current climate state

    • Began `2.5 million years ago in the northern hemisphere; as early as 30 million years ago in Antarctica

    • Evidence: Glacial Sediments and rock formations show ice sheets reached as far south as New York and Ireland

    • This climate phase has shaped human evolution and enabled civilization during the current interglacial period

Major Factors that affect climate change

  • Climate change is driven by a combination of natural and human factors

  • Even as global temperatures rise, some regions may experience short-term cooling

  • This results from shifts in oceanic and atmospheric circulation patterns.

Radiative Forcing

  • It refers to an imbalance between incoming solar radiation and outgoing heat at the top of Earth’s atmosphere.

  • Even a small imbalance can cause significant climate shifts

  • Measured by satellites, Earth is currently near radiative balance— but trends matter

Key factors in radiative forcing:

  • The Sun

    • Primary energy source

    • Radiant output is nearly constant, but modulated by sunspot cycles

  • Albedo (reflectivity)

    • Influences how much solar energy is reflected vs. absorbed

    • High albedo: Snow, ice, clouds, aerosols —> reflect more

    • Low albedo: Oceans, forests, dark soils —> absorb more

    • “A minor change in reflection or solar input can trigger major shifts in climate over time”

Earth’s Energy Balance

  • Earth’s Climate system:

    • Balance is regulated by energy exchanges across

      • Atmosphere (Air)

      • Hydrosphere (Water)

      • Biosphere (Living things)

      • Cryosphere (Ice)

      • Lithosphere (Rock)

Radiative Feedback

  • Balancing the Climate System; net radiative forcing is determined by the interaction between:

    • Climate drivers

      • Solar radiation

      • Volcanic activity

      • Anthropogenic GHG emissions

    • Climate Modulators

      • Ice & cloud cover

      • Atmospheric water vapor

      • Land/ocean GHG release

      • Erosion rates and surface reflectivity

  • Feedback mechanisms in the Climate System Positive Feedback (amplifies Warming): Melting Arctic Sea Ice

    • High-albedo ice replaces by low-albedo ocean

    • More solar energy absorbed

    • Further warming

  • Negative Feedback (Reduces Warming): Evaporation and Cloud Formation

    • Higher temperatures increase evaporation

    • More clouds reflect sunlight

    • Reduced warming impact

Timescale of Climate Change

Energy balance shifts can happen

  • in less than 10 years (e.g. volcanic eruptions)

  • Over centuries or millennia (e.g. ice sheet growth)

  • Over millions of years due to tectonic movement

Tectonics set the stage for long-term climate patterns by reshaping continents, oceans, and atmospheric circulation

Global tectonics and climate change 

  • Continental drift:

    • Plates shift ~2-3 cm a year (nail growth rate)

    • Alters ocean currents, wind patterns, and albedo

    • affects long-term climate and biodiversity

  • Example- Permian period (~250 million years ago)

    • Pangaea supercontinent formed

      • Vast interior regions became hot and dry

      • Life stressed —> high extinction rates

How continental Positions Shape Global Global Climate

  • Earth’s continents have drifted far from their arrangement in the supercontinent Pangaea

  • Today’s layout allows moisture to reach most land areas, except for deep continental interiors

  • With Antarctica over the south pole and the North pole surrounded by land, warm ocean currents are blocked from reaching the poles

  • This has led to extremely cold polar climates over the last 30 million years

  • Continents act as climate engines- they can create “refrigerators” (cold periods) or “ovens” (hot periods)

  • Current configuration= a planetary refrigerator, unlikely to change without external (human) influence

Earth’s orbit and Climate Change: Milankovitch Cycles

  • Earth’s orbit changes over 10,000 - 100,000 years, affecting seasonal solar radiation

  • Three key orbital changes

    • Obliquity- tilt of Earth’s axis

    • Precession- direction of the axis tilt

    • Eccentricity- Shape of Earth’s orbit

  • These are called Milankovitch cycles

  • When amplified by Earth’s climate system, they drive major climate shifts

  • Responsible for ice age cycles over the past 2.5 million years 

  • Without human impact, ice sheets may expand again in ~20,000 years

Ocean Circulation and Climate Interact

  • The global Ocean conveyor (Thermohaline Circulation):

    • Deep ocean circulation stores heat and GHG for centuries

    • Begins in the Arctic where cold, salty water sinks, flowing globally before resurfacing as warm currents

    • Acts as a climate buffer, moderating global temperatures

  • Ocean-atmosphere interaction

    • Operates on year-to-decade scales

    • Includes complex feedbacks among sun, volcanoes, atmosphere, oceans, clouds, aerosols, GHGs, and albedo

  • ESNO (El Niño-Southern Oscillation):

    • Occurs every 3-7 years

    • Causes climate extremes: droughts, heatwaves, floods

    • Example: in Cali, El Niño triggers intense storms and flood damage

Natural Climate Drivers vs. Recent Climate Change

  • NAO influence: The North Atlantic Oscillation affects winter conditions in the Northern hemisphere by shifting wind patterns

  • Volcanic impact: Mount Pinatubo’s 1991 eruption temporarily cooled Earth by half a degree C due to sulfate aerosols increasing albedo

  • Complex Interactions: Climate is shaped by natural cycles (e.g. ocean currents, volcanic activity, atmospheric pressure) that interact dynamically

  • Stability of Natural Factors: Over the past 10,000 years, tectonics, orbital cycles, and deep ocean currents have remained largely stable

  • Modern Warming: Current global temperature increases since the mid-20th century cannot be explained by natural factors alone